The distinction, then, between direct and circumstantial evidence is this. Direct or positive evidence is when a witness can be called to testify to the precise; fact which is the subject of the issue in trial; that is, in a case of homicide, that the party accused did cause the death of the deceased. Whatever may be the kind or force of the evidence, this is the fact to be proved. But suppose no person was present on the occasion of the death,—and of course no one can be called to testify to it,—is it wholly unsusceptible of legal proof? Experience has shown that circumstantial evidence may be offered in such a case; that is, that a body of facts may be proved of so conclusive a character, as to warrant a firm belief of the fact, quite as strong and certain as that on which discreet men are accustomed to act in relation to their most important concerns....

Each of these modes of proof has its advantages and disadvantages; it is not easy to compare their relative value. The advantage of positive evidence is, that you have the direct testimony of a witness to the fact to be proved, who, if he speaks the truth, saw it done; and the only question is, whether he is entitled to belief. The disadvantage is, that the witness may be false and corrupt, and the case may not afford the means of detecting his falsehood.

But in a case of circumstantial evidence where no witness can testify directly to the fact to be proved, you arrive at it by a series of other facts, which by experience we have found so associated with the fact in question, as in the relation of cause and effect, that they lead to a satisfactory and certain conclusion; as when footprints are discovered after a recent snow, it is certain that some animated being has passed over the snow since it fell; and, from the form and number of the footprints, it can be determined with equal certainty, whether it was a man, a bird, or a quadruped. Circumstantial evidence, therefore, is founded on experience and observed facts and coincidences, establishing a connection between the known and proved facts and the fact sought to be proved.[15]

Under the head of direct evidence, as I shall use the term, would fall the evidence of material objects: in an accident case, for example, the scar of a wound may be shown to the jury; or where the making of a park is urged on a city government, the city council may be taken out to see the land which it is proposed to take. Though such evidence is not testimony, it is direct evidence, for it is not based on reasoning and inference.

29. Direct Evidence. Direct evidence is the testimony of persons who know about the fact from their own observation: such is the testimony of the witnesses to a will that they saw the testator sign it, the testimony of an explorer that there are tribes of pygmies in Africa, the testimony of a chemist to the constituents of a given alloy, or of a doctor to the success of a new treatment. Every day of our lives we are giving and receiving direct evidence; and of this evidence there is great variety in value.

In the first place, no one should place too much reliance on his own casual observations. It is notorious that we see what we expect to see; and no one who has not deliberately set himself to observe the fact can realize how much of what he thinks is observation is really inference from a small part of the facts before him. I feel a slight tremor run through the house with a little rattling of the windows, and assume that a train has gone by on the railroad below the hill a hundred yards away: as a matter of fact it may have been one of the slight earthquake shocks which come every few years in most parts of the world. The mistakes that most of its make in recognizing people are of the same sort: from some single feature we reason to an identity that does not exist.

Of recent years psychologists have set themselves to getting some accurate facts as to this inaccuracy of human observation, and various experiments have been tried. Here is an account of one:

There was, for instance, two years ago in Goettingen a meeting of a scientific association, made up of jurists, psychologists, and physicians, all, therefore, men trained in careful observation. Somewhere in the same street there was that evening a public festivity of the carnival. Suddenly, in the midst of the scholarly meeting, the doors open, a clown in highly colored costume rushes in in mad excitement, and a negro with a revolver in hand follows him. In the middle of the hall first the one, then the other, shouts wild phrases; then the one falls to the ground, the other jumps on him; then a shot, and suddenly both are out of the room. The whole affair took less than twenty seconds. All were completely taken by surprise, and no one, with the exception of the president, had the slightest idea that every word and action had been rehearsed beforehand, or that, photographs had been taken of the scene. It seemed most natural that the president should beg the members to write down individually an exact report, inasmuch as he felt sure that the matter would come before the courts. Of the forty reports handed in, there was only one whose omissions were calculated as amounting to less than twenty per cent of the characteristic acts; fourteen had twenty to forty per cent of the facts omitted; twelve omitted forty to fifty per cent, and thirteen still more than fifty per cent. But besides the omissions there were only six among the forty which did not contain positively wrong statements; in twenty-four papers up to ten per cent of the statements were free inventions, and in ten answers—that is, in one fourth of the papers—more than ten per cent of the statements were absolutely false, in spite of the fact that they all came from scientifically trained observers. Only four persons, for instance, among forty noticed that the negro had nothing on his head; the others gave him a derby, or a high hat, and so on. In addition to this, a red suit, a brown one, a striped one, a coffee-colored jacket, shirt sleeves, and similar costume were invented for him. He wore in reality white trousers and a black jacket with a large red neck-tie. The scientific commission which reported the details of the inquiry came to the general statement that the majority of the observers omitted or falsified about half of the processes which occurred completely in their field of vision. As was to be expected, the judgment as to the time duration of the act varied between a few seconds and several minutes.[16]

Another type of cases in which our direct testimony would be valueless is legerdemain: we think that we actually see rabbits taken out of our neighbor's hat, or his watch pounded in a mortar and presently shaken whole and sound out of an empty silk handkerchief; and it is only by reasoning that we know our eyes have been deceived.

It is obvious, therefore, that to question a man's evidence is not always to call him a liar; in most cases it is rather to question the accuracy of his inferences from such part of the facts as he actually grasped. In science no important observation is accepted until the experiments have been repeated and checked by other observers. Indeed, most of the progress of science is due to the repetition of experiments by observers who notice some critical phenomena which their predecessors have missed.

With this qualification, that human observation is always fallible, good direct evidence is on the whole the most convincing evidence that you can use. If you can establish a fact by the mouths of trustworthy witnesses, making your readers recognize that these witnesses had good opportunities of observation and a competent knowledge of the subject, you will generally establish your point. In case of an accident in a street car it is the custom of many companies to require their conductors to take down immediately the names of a few of the most respectable-looking of the passengers, who may be called as witnesses in case of a lawsuit. All the observations of science, and most of the facts brought before juries in courts of law, as well as the multitude of lesser and greater facts which we accept in everyday life, get their authority from this principle.

In the arguments of school and college you may not make much use of direct evidence, for they do not often turn on single, simple facts. Even here, however, cases arise where you must call in the direct testimony of witnesses. If you were arguing that secret societies should be abolished in a certain school, and wished to show that such societies had led to late hours, playing cards for money, and drinking, you would need direct evidence. If you were arguing that the street railroad company of your city should be obliged to double track a certain part of its line, you would need direct evidence of the delays and crowding of cars with a single track.

When you are using direct evidence you should make it clear that the person from whom it comes is a competent witness, that he has been in a position to know the facts at first hand, and that, if necessary, he has had the proper training to understand their meaning. In the case of an automobile accident a man who had never run a car would not be the best sort of witness as to the actions of the chauffeur, nor a man who had never sailed a boat as to what happened in a collision between two sailboats. In a scientific matter the observations of a beginner would not carry weight as against those of a man who had used a microscope for many years.

The witness, too, must be shown to be free from bias, whether practical or theoretical. It is a well-known fact that men differ greatly in the clearness of their eyesight in observing the stars, and that men who are gifted with exceptional eyesight may make valuable discoveries with inferior instruments; but if such a man has espoused a theory, say, as to the nature of the rings of Saturn, and is known to defend it passionately, his evidence concerning what he had seen is bound to be somewhat discounted.

Even official reports cannot be trusted without scrutiny.

The fact is that many things conspire to make an official report constrained and formal. There is the natural desire of every man to put the best face on things for himself as he sets his case before the government and the world; subordinates must be let off leniently; you must live with them, and it impairs comfort to have them sullen. To make a statement unpleasant to a superior might be construed as insubordination. The public welfare makes it imperative to tell a flattering tale. The temptation is constant to tell not quite the whole truth, and nothing but the truth. There are important suppressions of fact in the official records, none more so, perhaps, than as regards Chancellorsville.[17]

If you happen to be dealing with a historical matter, where the testimony comes from a more or less remote past, and the evidence is scrappy and defective, you must be still more careful.

The great English historian, the late S.R. Gardiner, in his examination of the evidence on the Gunpowder Plot of 1605, wrote as follows about the difficulties of dealing with historical evidence:

It seems strange to find a writer so regardless of what is, in these days, considered the first canon of historical inquiry, that evidence worth having must be almost entirely the evidence of contemporaries who are in a position to know something about that which they assert. It is true that this canon must not be received pedantically. Tradition is worth something, at all events when it is not too far removed from its source. If a man whose character for truthfulness stands high, tells me that his father, also believed to be truthful, seriously informed him that he had seen a certain thing happen, I should be much more likely to believe that it was so than if a person, whom I knew to be untruthful, informed me that he had himself witnessed something at the present day. The historian is not bound, as the lawyer is, to reject hearsay evidence, because it is his business to ascertain the truth of individual assertions, whilst the lawyer has to think of the bearing of the evidence not merely on the case of the prisoner in the dock, but on an unrestricted number of possible prisoners, many of whom would be unjustly condemned if hearsay evidence were admitted. The historian is, however, bound to remember that evidence grows weaker with each link of the chain. The injunction, "Always leave a story better than you found it," is in accordance with the facts of human nature. Each reporter inevitably accentuates the side of the narrative which strikes his fancy, and drops some other part which interests him less. The rule laid down by the late Mr. Spedding, "When a thing is asserted as a fact, always ask who first reported it, and what means he had of knowing the truth," is an admirable corrective of loose traditional stories.

A further test has to be applied by each investigator for himself. When we have ascertained, as far as possible, on what evidence our knowledge of an alleged fact rests, we have to consider the inherent probability of the allegation. Is the statement about it in accordance with the general workings of human nature, or with the particular working of the nature of the persons to whom the action in question is ascribed? Father Gerard,[18] for instance, lavishly employs this test. Again and again he tells us that such and such a statement is incredible, because, amongst other reasons, the people about whom it was made could not possibly have acted in the way ascribed to them. If I say in any of these cases that it appears to me probable that they did so act, it is merely one individual opinion against another. There is no mathematical certainty on either side. All we can respectively do is to set forth the reasons which incline us to one opinion or another, and leave the matter to others to judge as they see fit.

It will be necessary hereafter to deal at length with father Gerard's attack upon the evidence, hitherto accepted as conclusive, of the facts of the plot. A short space may be allotted to the reasons for rejecting his preliminary argument, that it was the opinion of some contemporaries, and of some who lived in a later generation, that Salisbury contrived the plot in part, if not altogether. Does he realize how difficult it is to prove such a thing by any external evidence whatever? If hearsay evidence can be taken as an argument of probability, and in some cases of strong probability, it is where some one material fact is concerned. For instance, I am of opinion that it is very likely that the story of Cromwell's visit to the body of Charles I on the night after the king's execution is true, though the evidence is only that Spence heard it from Pope, and Pope heard it, mediately or immediately, from Southampton, who, it is alleged, saw the scene with his own eyes. It is very different when we are concerned with evidence as to an intention necessarily kept secret, and only exhibited by overt acts in such form as tampering with documents, suggesting false explanation of evidence, and so forth. A rumor that Salisbury got up the plot is absolutely worthless; a rumor that he forged a particular instrument would be worth examining, because it might have proceeded from some one who had seen him do it.[19]

While it is rare to find a man of whom it may justly be said that there is no partition between his memory and his imagination, yet there are few of us who can be sure of facts in past matters which touch our feelings. We cannot help to some degree reconstructing events as they fade away into the past: we forget those parts of an event which did not at the time sharply touch our imagination, and those which did move us take on an overshadowing importance. Therefore the further away the events which the evidence is to reconstruct, the more care we must take to scrutinize it to see if there are signs of bias.

To test the value of direct evidence, therefore, as to single and simple facts, consider whether the evidence comes from a specifically named source, whether there is any likelihood that the witness may have been honestly deceived in his observation, whether he had a good opportunity to know the facts and a sufficient knowledge of the subject about which he is giving evidence, and, finally, whether he was reasonably free from bias in the matter.

Whenever you use direct evidence, however, it must be direct. To assert that "every one knows that secret societies in a certain school have led to immoral practices," is not direct evidence, nor to declare that "the best authorities in the city are agreed that the company should lay double tracks on a certain street." Such assertions are apt to be the most roundabout sort of hearsay. Try cross-examining the next man you hear make this kind of sweeping assertion, in order to see what he really knows of the facts, and you will soon find how recklessly such assertions are made. You constantly hear grave statements of facts whose ultimate basis is the imagination of some enterprising newspaper reporter; yet careful and truthful people pass them on as if they were indubitable.

The news columns of the papers are largely written by young fellows just out of high school, who will declare the whole gospel on subjects with which they have a half hour's acquaintance, yet most people never question their statements. The printed page, whether of a hook, a magazine, or a newspaper, casts a spell on our judgment. Such floating assertions, with no one to father them, are of no value whatever. If you have to use statements in a newspaper as direct evidence, either take them from a newspaper which is recognized as careful about facts, or else look up the matter in two or three papers, and show that their testimony agrees.

On the other hand, a specific name, with a specific reference to volume and page, will go a long way to give your readers confidence in the evidence you adduce. And rightly so, for one man with a name and address is worth hundreds of unnamed "highest authorities"; and the more specifically you refer to him and to his evidence, the more likely you will be to win over your audience to your view.

A famous and effective example of the use of specific names to give authority to an argument, and the incidental refutation of a vague and loose assertion, is found in Lincoln's address at Cooper Institute, in the first part of which he took up Senator Douglas's statement that "our fathers, when they framed the government under which we live, understood this question just as well as, and even better than, we do now," with the implication that they intended to forbid the federal government to control slavery in the federal territories. Lincoln showed that "our fathers who framed the government under which we live" must be the makers of the Constitution: and then he proceeded to show just what action each one of them, so far as record had been preserved, had taken on the question. Here is a passage from his argument:

The question of Federal control in the Territories seems not to have been directly before the convention which framed the original Constitution; and hence it is not recorded that the "thirty-nine," or any of them, while engaged on that instrument, expressed any opinion on that precise question.

In 1789, by the first Congress which sat under the Constitution, an act was passed to enforce the ordinance of 1787, including the prohibition of slavery in the Northwestern Territory. The bill for this act was reported by one of the "thirty-nine"—Thomas Fitzsimmons, then a member of the House of Representatives from Pennsylvania. It went through all its stages without a word of opposition, and finally passed both branches without ayes and nays, which is equivalent to a unanimous passage. In this Congress there were sixteen of the thirty-nine fathers who framed the original Constitution. They were John Langdon, Nicholas Gilman, William S. Johnson, Roger Sherman, Robert Morris, Thomas Fitzsimmons, William Few, Abraham Baldwin, Rufus King, William Paterson, George Clymer, Richard Bassett, George Read, Pierce Butler, Daniel Carroll, and James Madison.

This shows that, in their understanding, no line dividing local from Federal authority, nor anything in the Constitution, properly forbade Congress to prohibit slavery in the Federal territory; else both their fidelity to correct principle, and their oath to support the Constitution, would have constrained them to oppose the prohibition.

Again, George Washington, another of the "thirty-nine," was then President of the United States and as such approved and signed the bill, thus completing its validity as a law, and thus showing that, in his understanding, no line dividing local from Federal authority, nor anything in the Constitution, forbade the Federal Government to control as to slavery in Federal territory.

No great while after the adoption of the original Constitution, North Carolina ceded to the Federal Government the country now constituting the State-of Tennessee; and a few years later Georgia ceded that which now constitutes the States of Mississippi and Alabama. In both deeds of cession it was made a condition by the ceding States that the Federal Government should not prohibit slavery in the ceded country. Besides this, slavery was then actually in the ceded country. Under these circumstances, Congress, on taking charge of these countries, did not absolutely prohibit slavery in them. But they did interfere with it—take control of it——even there, to a certain extent. In 1798 Congress organized the Territory of Mississippi. In the act of organization they prohibited the bringing of slaves into the Territory from any place without the United States, by fine, and giving freedom to slaves so brought. This act passed both branches of Congress without yeas and nays. In that Congress were three of the "thirty-nine" who framed the original Constitution. They were John Langdon, George Read, and Abraham Baldwin. They all probably voted for it. Certainly they would have placed their opposition to it upon record if, in their understanding, any line dividing local from Federal authority, or anything in the Constitution, properly forbade the Federal Government to control as to slavery in Federal territory.

In the end this exact statement of names, for which he had prepared himself with such laborious care, enabled Lincoln to sum up with absolute conclusiveness:

The cases I have mentioned are the only acts of the "thirty-nine," or of any of them, upon the direct issue, which I have been able to discover.

To enumerate the persons who thus acted as being four in 1784, two in 1787, seventeen in 1789, three in 1798, two in 7804, and two in 1819-1820, there would be thirty of them. But this would be counting John Langdon, Roger Sherman, William Few, Rufus King, and George Read each twice, and Abraham Baldwin three times. The true number of those of the "thirty-nine" whom I have shown to have acted upon the question which, by the text, they understood better than we, is twenty-three, leaving sixteen not shown to have acted upon it in any way.

Here, then, we have twenty-three out of our thirty-nine fathers "who framed the government under which we live," who have, upon their official responsibility and their corporal oaths, acted upon the very question which the text affirms they "understood just as well, and even better, than we do now"; and twenty-one of them—a clear majority of the whole "thirty-nine"—so acting upon it as to make them guilty of gross political impropriety and willful perjury if, in their understanding, any proper division between local and Federal authority, or anything in the Constitution they had made themselves, and sworn to support, forbade the Federal Government to control as to slavery in the Federal territories. Thus the twenty-one acted; and, as actions speak louder than words, so actions under such responsibility speak still louder.

When you come to evidence about a large and complex state of affairs, which is the kind of fact that so many of the arguments of practical life deal with, though you will still be dealing with a fact, yet the very nature of the fact changes the value and the character of your evidence. It is a comparatively simple matter to determine whether a certain woman faced forward or backward as she was getting off a street car, or whether the eggs of a sea urchin do or do not begin to germinate under the influence of a certain chemical substance; but it is far from simple to determine whether a free elective course has or has not inured to greater intelligence and cultivation in the graduates of a certain college, or whether the graduates of another college where the classical course is maintained have keener and more flexible minds and more refined tastes as a result of their study of the classics. In such cases as these the citing of direct evidence brings on you difficulties of a different kind from those you face when you are establishing a single, simple fact. Here you will usually depend on two main sources of evidence: statistics, and the evidence of recognized authorities on the subject.

30. Statistics. Statistics, which are collections of figures, are notoriously treacherous. On many important subjects, such, for example, as the practical effect of the elective system, it is impossible to get them; and on many other subjects, such as the effects of a protective tariff, they must be had in so enormous masses, if they are to be trusted at all, that only profound students can handle them. Where the facts are complicated, and interests are tangled, moreover, many sets of figures may enter into the question, as notably in the case of a tariff; so clearly is this difficulty now recognized that Congress has authorized a tariff board made up of distinguished students of economics and men of long experience in dealing with tariff matters to collect and study the facts and make recommendations based on them. Similarly, with the investigation into the liquor question made fifteen years ago by the Committee of Fifty: the whole question had been so tangled by assertion and counter-assertion that it became desirable to have an investigation into the facts by men of recognized ability and impartiality.[20]

In general, to use statistics safely you need a wide acquaintance with a subject, especially where the question is in any way mixed up with men's feelings, whether through politics or not. All the statistics we have make dead against great armaments and preparation for war; yet while human nature is what it is, necessary prudence seems to require every nation of any size to have them. A very little human nature will upset a very great body of statistics. Furthermore, in most human affairs results are produced by a multiplicity of causes; and though statistics may throw light on three quarters of all the causes that are potent in any given case, yet the other quarter which are irreducible to definite statement may wholly alter the result. If you are using statistics in your argument, therefore, as evidence of some large and complex fact, you should usually justify them to some extent by showing that there are no counteracting forces which they do not cover.

With this precaution, however, statistics are the foundation of most arguments on large questions. If you were arguing in favor of the purchase of local waterworks, you would present figures showing the number of houses using the public water supply, the rates paid, the profits of the company, the exact points at which public control could work economies. If you were arguing for a rule that no man shall play on a university team until he has been registered a year at the university, you would need statistics to show how many men would be affected by the rule. If you were arguing for a single session at a school instead of two, you would show exactly how many students in the school live more than a mile away from the building. In every case where statistics can be presented in such a way as to make clear that they fairly cover the ground, they are most valuable evidence. They give the argument the effect of being founded on a rock. If it be obvious that the statistics have been freshly gathered, and are not merely casual and second-hand gleanings, they have still greater effect, for then they have a secondary force in testifying to the personal knowledge that the witness has of the subject. We shall see later the danger of the fallacy of generalizing on too narrow a basis: a generalization based on a good body of statistics runs no danger of this fallacy.

31. The Opinion of Recognized Authorities. The other chief source of evidence to establish a fact which consists of a large and complex state of affairs is the opinion of recognized authorities on the subject. The strength of such evidence depends on whether the audience will accept the person you cite as having authority on the matter. Most of us read some newspaper or periodical in the opinions of which we have confidence, because they seem to be based on investigation and competent knowledge. The annual report of the Secretary of the Treasury is excellent evidence on the state of the national finances. The reports of presidents of colleges are excellent evidence from authorities on such questions as the value of the elective system or the effect of raising the standard of admission. The report of a dean or of a schoolmaster on the value of organized athletics is effective if the audience knows that he likes out-of-door sports and takes time to see the games. Evidence drawn from an authority who is likely to be used by the other side is doubly effective, since your readers recognize that his competence is admitted.

If a man has given his life to the study of a subject and has published books that are of recognized authority, his evidence will be of especial weight. Mr. Bryce's opinion on all questions concerning a state of affairs in this country would be recognized at once as weighty, for he has given time and study to collecting the multitude of small facts which constitute the large fact. His opinion that political honesty is increasing with us has brought comfort to many good citizens who had grown despondent over the accounts of recurrent rascality in the newspapers and magazines. This is a typical case for the citation of authorities; for the facts are enormous in number, very widely scattered, and often contradictory. Only a man who has taken the pains to keep himself constantly informed, whose judgment has been trained by long consideration and comparison of the facts, and who is born with the judicial temperament can attain the authority of Mr. Bryce.

There will be cases on which you will have the right to put yourself forward as an authority, for on many subjects which fall within the range of undergraduates their knowledge is first-hand. On all questions of athletics, especially, an undergraduate is apt to have freshly in mind a considerable mass of facts. In the same way, on the results of certain requirements for admission to college, you can speak from recent experience. In matters concerning your own city, too, you may have original knowledge.

If you are going to put yourself forward as an authority, however, you must round out your knowledge of the facts by extending it beyond your own personal experience. If it is a question of entrance requirements, you cannot stop with your own experience, or even with that of your own class at school. You must go back to the records of a number of classes before and perhaps after your own, and talk them over with the principal of the school, to see whether there are any special circumstances which affect any of them. If you are arguing for or against a change in the present rules of football, you would have to go beyond the games of your own college team, and beyond those of the present season. If, for example, it were a question of amending the rules concerning the forward pass, you could not speak with full authority unless you had looked up the accounts of the principal games for two or three years at any rate. If you put yourself forward, then, as a witness on one of these cases of complicated facts, you must make it clear to your readers that you have a right to be considered such. If you have the right, it would be folly to hide your light under a bushel.

An example of the care which is taken by men who have made themselves authorities on their subjects is to be found in the following passage from President Eliot's address, "A Wider Range of Electives in College Admission Requirements."[21] Notice how broad a basis he lays for his conclusions both in facts and in the opinions of other authorities. What should be the grounds of a just valuation of all the subjects that can be presented at admission examinations which include numerous options?

That question introduces us to a difficult inquiry. It is, of course, not an intelligent method to attribute a value to each subject in accordance with the time devoted to the examination in that subject. What clue have we toward a better mode of determining the value which ought to be attributed to each of the numerous electives, when the young men cannot present all the permitted subjects, and hardly three fifths of them, indeed, if the range is adequately widened? I believe that the best criterion for determining the value of each subject is the time devoted to that subject in schools which have an intelligent program of studies. The Committee of Ten[22] examined the number of subjects used in about two hundred of the best secondary schools in this country, and the time-allotments for the several subjects. They found a great variety of practice as to both selection of subjects and time-allotments. You can hardly say that there is an accepted time-allotment in these secondary schools for any subject—not even for the old traditional subjects. The time-allotments differ widely in different parts of the country, and even in different schools in the same part of the country. If, then, we are to determine by school time-allotments the valuations of the different subjects, prescribed and elective, which may enter into admission examinations, we must have some sort of standard programs for secondary schools. At present (1896) I know no programs which can answer that purpose, except the provisional programs of the Committee of Ten. They may fairly be said to be the best-studied programs now before the country, and to represent the largest amount of professional consent, simply because they are the result of the work, first, of ninety school and college teachers, divided into nine different conferences by subject, and secondly, of ten representative teachers combining and revising the work of the conferences, with careful reference to the present condition of American schools.

32. Indirect Evidence. The term "indirect evidence" may be used for all evidence as to fact in which reasoning consciously plays a part. Without it we should be helpless in large regions of our intellectual life, notably in science and history, and constantly in everyday life. Clearly the line between direct and indirect evidence is vague and uncertain; it is one of the first things learned in psychology that our perceptions and judgments of things about us are almost never based exclusively on the testimony of our senses, and that we are all the time jumping to conclusions from very partial observations.

Professor Muensterberg gives the following example from his own experience of this unintentional substitution of indirect evidence for direct:

Last summer I had to face a jury as witness in a trial. While I was with my family at the seashore my city house had been burglarized and I was called upon to give an account of my findings against the culprit whom they caught with part of the booty. I reported under oath that the burglars had entered through a cellar window, and then described what rooms they had visited. To prove, in answer to a direct question, that they had been there at night, I told that I had found drops of candle wax on the second floor. To show that they intended, to return, I reported that they had left a large mantel clock, packed in wrapping paper, on the dining-room table. Finally, as to the amount of clothes which they had taken, I asserted that the burglars did not get more than a specified list which I had given the police.

Only a few days later I found that every one of these statements was wrong. They had not entered through the window, but had broken the lock of the cellar door; the clock was not packed by them in wrapping paper, but in a tablecloth; the candle droppings were not on the second floor, but in the attic; the list of lost garments was to be increased by seven more pieces; and while my story under oath spoke always of two burglars, I do not know that there was more than one.[23]

Constantly in everyday life we make offhand assertions in the full belief that we are giving direct evidence, when as a matter of fact we are announcing inferences. The distinction is of importance in many ways, and not least as a means of avoiding heat in argument; for to question a man's inference is much less likely to make him angry than to deny his statement of fact.

For the practical purposes of argument we may let the distinction between observation and inference, and consequently that between direct and indirect evidence, turn on whether the inference is a conscious and readily distinguishable part of the judgment or not. Though bringing to light an unconscious inference is often an essential part of the detection of false reasoning, where there is no such practical consequence, we need not be too curious here about the line between direct observation and inference from observation. For the rough and ready purposes of everyday arguments it is exact enough to say that where you recognize that you are basing your conclusion as to a fact on some process of reasoning, then you are resting on indirect evidence; where you do not recognize the inference without reflection, you are resting on direct evidence.

In the following discussion of reasoning I shall sometimes be dealing with proving a fact, sometimes with arguing forward to a policy. In many cases the two processes are practically identical, for if the fact is established the policy follows as a matter of course: in these cases, therefore, for the sake of convenience I shall use the terms interchangeably, and keep them separate only where there is danger of confusion.

33. Reasoning. Though the various forms of reasoning and the principles which they follow are the concern rather of psychology and logic than of a practical work on the writing of arguments, yet these sciences help us to understand the processes of the mind by which we convince first ourselves, and then other people, of the existence of facts, when for one reason or another direct testimony is wanting. Psychology describes the processes of reasoning as part of the activity of the mind, analyzes them into their parts, and shows their working. Logic is concerned rather with the forms of reasoning: its aim is to establish principles and rules the application of which will insure correct reasoning.

I shall first briefly and very simply sketch the underlying nature of the reasoning process as it is described by psychologists; then I shall pass on to a practical application of the principles thereby attained; next I shall set forth a few of the simplest and clearest of the processes of reasoning which have been worked out by logic; and, finally, I shall discuss each few of the best-recognized forms of false reasoning. From both the psychological description and the rules of logic we shall derive practical suggestions for establishing facts which may be needed in an argument.

The essential feature of the process of reasoning is that it proceeds from like to like, by breaking up whole facts and phenomena, and following out the implications or consequences of one or more of the parts.[24] For example, if I infer, when my dog comes out of a barnyard with an apologetic air, and with blood and feathers on his mouth, that he has been killing a hen, I am breaking up the whole phenomenon of the dog's appearance, and paying attention only to the blood and feathers on his head; and these lead me directly to similar appearances when I have caught him in the act. If I reason, Every student who can concentrate his attention can learn quickly, George Marston has a notable power of concentration, Therefore George Marston can learn quickly, I again break up the abstraction student, and the concrete fact George Marston, and pay attention in each to the single characteristic, concentration of attention. Thus by means of these similar parts of different wholes I pass from the assertion concerning the class as a whole to the assertion concerning the concrete case. This process first of analysis and then of abstraction of similars is the essential part of every act of reasoning.

In intuitive or unreasoned judgment, on the other hand, we jump to the conclusion without analyzing the intermediate steps. If I say, I have a feeling in my bones that it will rain to-morrow, or, it is borne in on me that our team will win, the sensations and ideas that I thus lump together are too subtle and too complex for analysis, and the conclusion, though it may prove sound, is not arrived at by reasoning. The difference between such intuitive and unreasoned judgments, and reasoning properly so called, lies in the absence or the presence of the intermediate step by which we consciously recognize and choose out some single attribute or characteristic of the fact or facts we are considering, and pass from that to other cases in which it occurs.

The skill of the reasoner therefore consists of two parts: first, the sagacity to pick out of the complex fact before him, the attribute or characteristic which is significant for his present purpose; and second, the large knowledge of the subject which will enable him to follow it into other cases in which it occurs with different circumstances, or, in other words, to follow a similarity through diverse cases. Darwin's great achievement in establishing the principle of evolution lay first in the scientific sagacity which flashed home on him, after years of patient study, that the one common fact in all the multitude of plants and animals is that in the struggle for existence by which all living beings persist, those who are best fitted to their circumstances survive; and second, in his rich knowledge of the world of nature, which made it possible for him to follow out this characteristic in all kinds of plants and animals, and so to reach the general law. But whether it be so world-sweeping a conclusion as his, or my conclusion that my dog has killed a hen, the process is the same: analysis or breaking up of the complex fact, and following out the consequences or implications of some selected part of it into other cases.

All reasoning thus reduces itself in the end to a process of passing from like to like: we notice that the present case is like other cases which we already know: then, since these cases have always in the past been accompanied by certain circumstances or consequences, we believe that the present case will also show these same circumstances or consequences. Whenever my dog has killed when the cases have been similar in the blood and feathers on his mouth; in this case he has blood and feathers on his mouth; therefore he must have killed a hen. Individual plants and animals survive which are fitted to their environment by special characteristics, and those which are not so fitted die; species of plants and animals, as well as individuals, show special adaptation to their environment; therefore species have survived through the same process of natural selection.

It follows that reasoning, whether it results in a general law or in concrete judgment, depends on the assumption that nature—and in nature we mean here the whole universe as we know it is uniform; that there are ties between facts which make it possible for us to be certain that if a given fact occurs, then another fact always occurs with it as an effect, or as a cause, or connected with it in some other manner. Without this certainty of the uniformity of things there would be no reasoning, and therefore no argument from indirect evidence. Huxley sets forth this fundamental truth clearly and impressively at the beginning of the first of his "Lectures on Evolution" (see p. 234).

For practical purposes the various types of this inference from similarity can be conveniently thrown into three groups. As will be obvious, there is no fixed and impassable line between them.

"If an inference relies upon a resemblance that is newly seen, rare, or doubtful, it is called an inference from analogy; if it is made upon the basis of an established classification, it is called a generalization; if it involves a variety of resemblances so combined as to bear upon a single point, it is usually or frequently called an inference from circumstantial evidence."[25]

I will take up each of these types and show how we use them in the practical work of argument. It will be seen that they vary greatly in certainty of results.

34. Reasoning from Analogy. Analogy in its most tenuous form is weak as a basis for an actual inference, though it is often effective as a means of expressing an intuitive judgment where the reasons are too subtle and diffused for formal explanation. When Lincoln in the middle of the Civil War said that men do not swap horses while they are crossing a stream, the analog, though subtle, was felt to be real. Popular adages and proverbs are common modes of expressing such deep-lying analogies: for example, "Where there is smoke there is fire"; "The slothful man saith, There is a lion in the way." Poetry too is full of these subtle, pregnant similarities which link things in some one aspect, but fail for all others.

To die; to sleep; No more; and by a sleep to say we end The heart-ache and the thousand natural shocks That flesh is heir to. 'Tis a consummation Devoutly to be wish'd. To die; to sleep;— To sleep? Perchance to dream! Ay, there's the rub; For in that sleep of death what dreams may come, When we have shuffl'd off this mortal coil, Must give us pause.

But, as in this case of Hamlet's, poetical analogies will not bear much strain; the aspect in which the similarity holds is usually the only aspect the two cases have in common, and to take poetry as a precise formulation of fact is to sin against both humor and sound reasoning.

In daily life we are constantly reasoning by analogy. If you argue that a certain man who has been successful at the head of a railroad will therefore make a good president for a college because that also is a complex institution, or that because self-government has worked well in a certain school it will probably work well in a college, or that because a friend has been cured of sleeplessness by taking a walk just before going to bed therefore everybody who sleeps badly can be cured in the same way,—in all these cases you are reasoning by analogy. In each case it will be noticed you would pass from a similarity which exists in a single case or in a small number of cases to the conclusion. The reasoning is sound, however, only in so far as the similarity bears on the actual purpose in hand: in the first example, if the success of the railroad president arises from the power of understanding men and of philosophic insight into large problems, the reasoning will probably be valid; in the last example, if applied to insomnia due to overwork, it might be bad.

In practical affairs it is easy to find examples of reasoning from analogy, especially in arguments of policy. The first trial of city government by commission depended on such reasoning: when Galveston, Texas, was devastated by a storm it was reasoned that in business matters a small body of picked men with absolute powers are most efficient in an emergency, and that since the reconstruction of the city was essentially a matter of business, such a body would best meet the emergency. So the extension of commission government in other states at first followed reasoning by analogy: government by commission worked well in Galveston; it would probably work well in Des Moines. In the same way with the arguments for a parcels post: they proceed from the analogy of the present postal service, which has been successful so far as it goes, and from the success of the parcels post in almost all the countries of Europe. If you were arguing that "Association" (or "soccer") football should be made one of the major sports at your college, you would reason from the analogy of its great popularity with Englishmen all over the world that it would also probably be popular in America.

When you use the argument from analogy, however, you must make sure that the similarity between the two cases runs to the point you wish to establish. In the following extract from an argument in favor of commission government for all cities, the author explicitly limits his reasoning from the analogy of Washington to the point of the extension of the system to large cities.

If we look for successful governments by commission in this country, it is not difficult to find them in our largest cities. The city of Washington is governed by a small commission, and is acknowledged to be one of our best-governed cities. While this commission originated in an entirely different way from that of the commission form of government, successful administration under its rule is a valid answer to the argument that small commissions are suited only to the administration of small cities.[26]

Whenever you use this type of reasoning, it is wise thus to limit its bearing. If in an argument in favor of allowing secret societies in a high school you rely on the analogy of college life, take pains to show that the resemblance covers the social life of a school. If you were arguing that your city should establish a municipal gymnasium, and relied on the reasoning from the analogy of a family, in which all the members have a direct interest in the health of the others, show that this interest has practical grounds of welfare, and does not rest wholly on affection. In every case, unless the limits of the analogy are obvious, specify them in order to carry your readers safely with you.

35. False Analogy. A peculiar danger of the argument from analogy is the fallacy which is known as false analogy, or reasoning to a conclusion which the similarity does not support. Arguments in which there are many figures of speech, especially when the style is at all florid, are apt to slop over into this fallacy. To liken education to the unfolding of a flower is all very well, if you do not go on to argue that because the lily of the field neither toils nor spins, therefore a child should do no work in school. It is said that M. Stolypin, the late premier of Russia, once half apologized In the Duma for the slowness of his reforms, saying that he Was like a man shooting with a flintlock musket; to which one of the Liberal members replied that it was not a question of weapons, but of aim, and that if his Excellency was to go on shooting at the people, it would be better if he went on using flintlocks. Under the auspices of the Carnegie Institution an expert in business administration made an inquiry into the methods of teaching and research in physics at various American universities, and made recommendations based on the conduct of business establishments. A professor of physics in answer showed in how many ways the analogy between a business concern, whose end is profit, and a physical laboratory, whose end is the advancement of knowledge, is false and misleading. The expert had suggested a general research board to correlate researches; the professor cited the cases of Airy, the astronomer royal of England, who by his dominating position held back astronomical research in England for a generation, and of Sir Humphry Davy, who discouraged the work of Faraday, when the latter was his assistant.

The expert suggested that apparatus could be passed on from one investigator to another: the professor replied that few men can use apparatus designed for some one else's purpose, and that the cost of reconstruction would exceed the cost of new machines. In short, he completely riddled the argument from analogy set up by the expert.[27]

A notable example of conclusive refutation of an argument based on a false analogy is to be found in William James's Ingersoll Lecture on Immortality. He took up the ordinary argument against the immortality of the soul, which, starting from the accepted physiological and psychological formula, "Thought is a function of the brain," reasons that therefore when the brain dies and decays, thought and consciousness die, too.

This, then, is the objection to immortality; and the next thing in order for me is to try to make plain to you why I believe that it has in strict logic no deterrent power. I must show you that the fatal consequence is not coercive, as is commonly imagined; and that, even though our soul's life (as here below it is revealed to us) may be in literal strictness the function of a brain that perishes, yet it is not at all impossible, but on the contrary quite possible, that the life may still continue when the brain itself is dead.

The supposed impossibility of its continuing comes from too superficial a look at the admitted fact of functional dependence. The moment we inquire more closely into the notion of functional dependence, and ask ourselves, for example, how many kinds of functional dependence there may be, we immediately perceive that there is one kind at least that does not exclude a life hereafter at all. The fatal conclusion of the physiologist flows from his assuming offhand another kind of functional dependence, and treating it as the only imaginable kind.

When the physiologist who thinks that his science cuts off all hope of immortality pronounces the phrase, "Thought is a function of the brain," he thinks of the matter just as he thinks when he says, "Steam is a function of the teakettle," "Light is a function of the electric circuit," "Power is a function of the moving waterfall." In these latter cases the several material objects have the function of inwardly creating or engendering their effects, and their function must be called productive function. Just so, he thinks, it must be with the brain. Engendering consciousness in its interior, much as it engenders cholesterin and creatin and carbonic acid, its relation to our soul's life must also be called productive function. Of course, if such production be the function, then when the organ perishes, since the production can no longer continue, the soul must surely die. Such a conclusion as this is indeed inevitable from that particular conception of the facts.

Rut in the world of physical nature productive function of this sort is not the only kind of function with which we are familiar. We have also releasing or permissive function; and we have transmissive function.

The trigger of a crossbow has a releasing function: it removes the obstacle that holds the string, and lets the bow fly back to its natural shape. So when the hammer falls upon a detonating compound. By knocking out the inner molecular obstructions, it lets the constituent gases resume their normal bulk, and so permits the explosion to take place.

In the case of a colored glass, a prism, or a refracting lens, we have transmissive function. The energy of the light, no matter how produced, is by the glass sifted and limited in color, and by the lens or prism determined to a certain path and shape. Similarly, the keys of an organ have only a transmissive function. They open successively the various pipes and let the wind in the air chest escape in various ways. The voices of the various pipes are constituted by the columns of air trembling as they emerge. But the air is not engendered in the organ. The organ proper, as distinguished from its air chest, is only an apparatus for letting portions of It loose upon the world in these peculiarly limited shapes.

My thesis now is this: that, when we think of the law that thought is a function of the brain, we are not required to think of productive function only; we are entitled also to consider permissive or transmissive function. And this the ordinary psychophysiologist leaves out of account.[28]

The question of the validity of an analogy in reasoning is always, as here, whether the similarity on which the reasoning rests really runs between the two cases in hand, or is not merely a general resemblance expressed by some phrase or word which seems to mean more than it does. In other words, when you are testing an analogy, whether your own or an opponent's, make sure that the similarity is real for the present case. A picturesque figure of speech may add life to an argument, but it may also cover a gap in the reasoning.

36. Reasoning by Classification or Generalization. Obviously the strength of reasoning from analogy increases with the number of cases which you can point to as showing the similarity on which you rely, for you can then begin to generalize and classify.

Analogy expresses our natural tendency to assimilate the new to the old, to interpret what is strange and unfamiliar in the light of what we already know. It may therefore be described as classification in the making. The resemblances which guide us are called analogies so long as they are newly seen, rare, or doubtful; but as the number of cases increases, analogy passes by insensible stages into established classification.[29]

An excellent example of this transition may be seen in the present state of the argument in favor of commission government: at first, as we have seen, it depended chiefly on reasoning from analogy; by this time enough cities have adopted the plan to make it possible to classify them, and so reason by generalization.

Generalization and classification, it may be noted in passing, are two aspects of the same process of thought. When one passes from the individual facts to the larger fact which brings them together, as in the assertion, Members of the Phi Beta Kappa are good scholars, one makes a generalization; when one asserts of an individual the larger fact, as in the assertion, My brother is a good scholar (My brother belongs to the class Good Scholars), one makes a classification.

When a classification or generalization is constant and familiar, it brings forth, by the natural economy of language, a name for the class or the principle; "federation," "deciduous trees," "emotion," "terminal moraine," are all names of classes; "attraction of gravity," "erosion," "degeneration," "natural selection," are names of principles which sum up acts of generalization. Almost always these names begin as figures of speech, but where they are used accurately they have a perfectly exact meaning. Darwin has given some account of this process of language:

"It has been said that I speak of natural selection as an active power or deity, but who objects to an author speaking of the attraction of gravity as ruling the movements of planets? Every one knows what is meant by such metaphorical expressions, and they are almost necessary for brevity: so, again, it is difficult to avoid personifying the word 'Nature.' But I mean by Nature the aggregate action and product of many laws, and by laws the sequence of events as ascertained by us."[30]

When the facts intended to be meant by a phrase are thus carefully specified and delimited, the phrase ceases to be a figure of speech, and becomes the name of a class or of a principle.

Generalization and classification always take place for purposes of reasoning;[31] and reasoning which is dependent on them rests on the assumption that things are uniformly correlated in nature; when we throw things together into classes we assume that what is true for one member of a class, so far as it is a member of that class, is true to the same extent and for the purpose for which the class is made for all other members of that class.

In practice a large part of our reasoning is through generalization and classification; and as we have seen, it has a more substantial basis than when we rest on an analogy. If you hear that your brother has been elected to the Phi Beta Kappa, you reason from the generalization that all members of the Phi Beta Kappa are high scholars to the inference that your brother must have taken high rank. When I see a gang of carpenters knocking off work at four o'clock in the afternoon, I infer that they must belong to the union, because I know that unions as a class have established an eight-hour day. If you were arguing that the standards for graduation from your college should be raised, you would try to show that each year enough men are graduated with low intellectual attainments to make a class large enough to generalize from. If you were arguing that your city should establish a municipal gymnasium, you would try to show that of the boys and young men brought before the police courts for petty mischief and more serious offenses almost all have lacked the chance to work off their animal spirits in a healthy way. Wherever you can thus establish your special case in a class which has known characteristics or consequences, you can then apply the characteristics and consequences of the class to your special case.

Where the class is recognized as having definite characteristics or consequences, you can make your inference by showing that your case falls within the class. Sometimes the stress of your reasoning will come on making it clear that the consequence or characteristic on which your reasoning depends really belongs to the class. If, for example, you were arguing, as did the Class of '85 at Amherst College, that your college should return to something like the old-fashioned classical education, you would try to establish the fact that men who have had the old-fashioned classical education are as a rule characterized by intelligence, liberal culture, and open-mindedness. In such cases it is the generalization on which the class is based which is the difficult part of your task.

In general, however, if you can show your readers that the present case belongs in a class of cases which can be recognized as belonging together by virtue of definable characteristics, you have established an excellent foundation for an inference based on those characteristics.

37. Reasoning by Causal Relation. Reasoning by generalization rises greatly in certainty, however, whenever you can show the workings of cause and effect. If a college receives every year from a certain school a number of boys who are slack and lazy students, the dean of that college may come to generalize and expect most of the boys from that school to be poor timber. If, however, he finds that the master of the school will take and keep any boy who lives in the town, he is able to argue from this as a cause to the conclusion that the standards of the school are low, and then from these low standards as a cause to the poor quality of the graduates of the school.

Here is another example, from Professor James:

I am sitting in a railroad car, waiting for the train to start. It is winter, and the stove fills the car with pungent smoke. The brakeman enters, and my neighbor asks him to "stop that stove smoking." He replies that it will stop entirely as soon as the car begins to move. "Why so?" asks the passenger. "It always does," replies the brakeman. It is evident from this "always" that the connection between car moving and smoke stopping was a purely empirical one in the brakeman's mind, bred of habit. But if the passenger had been an acute reasoner ... [and had] singled out of all the numerous points involved in a stove's not smoking the one special point of smoke pouring freely out of the stove-pipe's mouth, he would probably ... have been immediately reminded of the law that a fluid passes more rapidly out of a pipe's mouth if another fluid be at the same time streaming over that mouth.[32]

Here the passenger's certainty that the smoking would stop would have been much increased if he had, as Professor James suggests, reasoned to the cause, instead of trusting to the brakeman's generalization from experience.

In scientific matters search for cause and effect the chief mode of progress. General Sternberg's article "Yellow Fever and Mosquitoes" (p. 251) is an admirable account of this advance from probability to certainty, which comes from demonstrating the necessary sequence which we call cause and effect. When Major Reed and his associates had shown that in cases where mosquitoes were kept away there was no yellow fever, but that in cases where infected mosquitoes were allowed to bite patients yellow fever followed, they turned the probability that mosquitoes were the transmitting agent of the fever into a certainty. Likewise with the glacial theory: it had already in the time of the elder Professor Agassiz been established that certain regions of northern Europe and America could be classed together by the occurrence of certain phenomena—rounded hills, ledges of rock smoothed off and marked with scratches running more or less north and south, deposits of clean gravel and sand, boulders of various foreign kinds of rock scattered over the surface of the country; when he showed that glaciers in their movements produce all these phenomena, he laid bare the cause of the phenomena, and so demonstrated with practical certainty the theory of the former existence of a huge glacial sheet in the northern hemisphere. Wherever you can show that your case not only belongs to a recognized class of cases, with recognized characteristics, but also that in those characteristics there is a necessary sequence of cause and effect, you have proved your point.

In the example above, of an argument for the establishment of a municipal gymnasium, if after showing that all the boys and young men who get into the courts have no normal and healthy way of working off their natural animal spirits, you can show that in places where through settlements or municipal action gymnasiums have been provided, the number of arrests of boys and young men has greatly fallen off, you have established the grounds for an inference of cause and effect which gives your argument a wholly new strength. In the case of the argument for a return to a classical course in a college, this sequence of cause and effect would be very difficult to establish, for here you would be deep down in the most complex and subtle region of human nature. Wherever it is possible, however, lead the inference from a classification or generalization on to an inference of cause and effect.

38. Induction and Deduction. Our next step is to consider how we get the generalizations on which we base so much of our reasoning. As we have seen, the science which deals with the making of them, with their basis, and with the rules which govern inferences made from them is logic.

Logicians generally distinguish between two branches of their science, inductive and deductive reasoning. In inductive reasoning we pass from individual facts to general principles; in deductive reasoning we pass from general principles to conclusions about individual facts. The distinction, however, draws less interest in recent times than formerly, and logicians of the present generation tend to doubt whether it has any vital significance.[33] They point out that in practice we intermingle the two kinds almost inextricably, that the distinction between facts and principles is temporary and shifting, and that we cannot fit some of the common forms of inference into these categories without difficult and complicated restatement.

Nevertheless, as deductive logic and inductive logic are ancient and time-honored terms which have become a part of the vocabulary of educated men, it is worth while to take some note of the distinction between them, I shall not attempt here to do more than to explain a few of the more important principles. I shall begin with inductive logic, since that is the branch which deals with the making of generalizations from individual fact, and therefore that which has most concern in the arguments of the average man in his passage through life.

39. Inductive Reasoning. In inductive reasoning we put individual facts and cases together into a class on the basis of some definable similarity, and then infer from them a general principle. The types of inductive reasoning have been reduced by logicians to certain canons, but these reduce themselves to two main methods, which depend on whether in a given piece of reasoning we start from the likeness between the instances or the differences between them. On these two methods, the method of agreement and the method of difference, hang all the processes of modern science, and most of our everyday arguments.

The method of agreement has been defined as follows:

If two or more instances of the phenomenon under investigation have only one circumstance in common, the circumstance in which alone all the instances agree is the cause (or effect) of the given phenomenon.[34]

A few examples, which might easily be multiplied, will show how constantly we use this method in everyday life. Suppose that a teacher is annoyed at somewhat irregular intervals by whispering and laughing in the back of the schoolroom, for which he can find no cause, but that presently he notices that whenever a certain pair of boys sit together there the trouble begins; he infers that these two boys are the cause of the trouble.

In the old days before it had been discovered that the germs of malaria are carried by mosquitoes, the disease was ascribed to a miasma which floated over low ground at night; and the innkeepers of the Roman Campagna, where malaria had almost driven out the population, urged their guests never to leave their windows open at night, for fear of letting in the miasma. In the lights of those days this was good reasoning by the method of agreement, for it was common observation that of all the many kinds of people who slept with their windows open most had malaria. We are constantly using this method in cases of this sort, where from observation we are sure that a single cause is at work under diverse circumstances. If the cases are numerous enough and diverse enough, we arrive at a safe degree of certainty for practical purposes. As the case just cited shows, however, the method does not establish a cause with great certainty. No matter how many cases we gather, if a whole new field related to the subject happens to be opened up, the agreement may be shattered.

The method of difference, which in some cases does establish causes with as great certainty as is possible for human fallibility, works in the opposite way: instead of collecting a large number of cases and noting the single point of agreement, it takes a single case and varies a single one of its elements. The method has been stated as follows:

If an instance in which the phenomenon occurs, and an instance in which it does not occur, have every circumstance in common save one, that one occurring only in the former; the circumstance in which alone the two instances differ, is the effect, or the cause, or an indispensable part of the cause, of the phenomenon.[35]

The principle is clearer and more apprehensible in the concrete example than in the abstract statement; as a matter of fact it is applied in every experimental search for a cause. The Agricultural College of New York, for example, in the course of certain experiments on apple orchards, bought an orchard which had not been yielding well, and divided it into halves; one half was then kept plowed and cultivated, the other half was left in grass; otherwise the treatment was the same. When the half which was kept cultivated gave a much larger yield than the other, it was safe to infer that the cultivation was the cause of the heavier yield. Dr. Ehrlich, the great German pathologist, is said to have tried six hundred and five different substances before he found one which would kill the germ of a certain disease; in each experiment he was using the method of difference, keeping the conditions the same in all except a single point, which was the addition of the substance used in that particular experiment. Wherever the conditions of an experiment can be thus controlled, the method of difference gives a very accurate way of discovering causes. With advancing knowledge a supposed cause may be in turn analyzed in such a way that each of its parts can be separately varied, in order to come more closely to the actual sequence involved.

It has been pointed out[36] that the two methods are really statements of what is required for the verification of a theory at two stages of its growth: when we are first getting a glimpse of a causal connection between two facts we collect all the cases in which they occur in as much variety as possible, to see if the connection is really universal; then, having established the universal sequence, we come to close quarters with it in a single critical instance, varying the conditions singly until we run down the one without which the effect cannot take place.

No neater and more illuminating example of this relation between the two methods and the successful working of them can be found than that in the article by General Steinberg, "Yellow Fever and Mosquitoes" (p. 251). In that case first Dr. Carlos Finlay of Havana, and then Dr. Sternberg himself, had become convinced by comparing many cases of yellow fever that there was some intermediate host for the bacillus that caused the disease. This conclusion they reached through the method of agreement. Dr. Finlay's experiments by the method of difference had failed, however, indisputably to establish the cause, since he did not see that it was necessary to allow the bacillus at least twelve days for incubation in the body of the mosquito. The final and definitive proof, which came through the splendid self-devotion of the surgeons in charge of the experiment and of certain enlisted men who volunteered to be made the subject of the experiment, was by the method of difference. These brave men allowed themselves to be exposed to mosquitoes which had already bitten patients suffering from the fever, and they promptly came down with the disease; one of them, Dr. Lazear, gave his life for his devotion to the cause of his fellow men. Then other men were exposed in a mosquito-proof room to clothes and other articles brought directly from yellow-fever patients, and showed no ill effects. Thus it was absolutely proved, though the bacillus itself had not been found, that yellow fever is carried by mosquitoes, and is not carried by ordinary contagion.

The unsuccessful experiments of Dr. Finlay and the later success of Major Reed show how science advances by refinement of analysis in the use of the method. The hypothesis on which the former worked was that all mosquitoes who had bitten a yellow-fever patient can carry the disease. Dr. Reed and his associates analyzed the phenomenon more closely and tried their experiments on the hypothesis that only mosquitoes who have lived twelve days after biting the patient are capable of passing on the disease. This refinement of analysis and observation is the chief mode of advance in the sciences which depend on experiment.

Scientific arguments, therefore, make constant use of both methods. Medical research frequently begins with the gathering of statistics from reported cases, and the theory or theories suggested by the method of agreement working on these facts leads to the application of the method of difference through some series of critical experiments. In general the conclusions of science where experiment cannot be used depend on the method of agreement, especially in the larger theories in biology and geology, where the lapse of unnumbered centuries is necessary to bring about changes. In physics, in chemistry, in medicine, on the other hand, critical experiments are generally possible, and so progress is by the method of difference. In such subjects as political science and government, where experiment is out of the question, one must depend chiefly on the method of agreement, except in such cases as will be mentioned below where a change in policy has the same effect as an experiment. Here, however, one must not forget that in all matters human the incalculable clement of human nature enters to complicate all results, and that emotion and feeling are always irrational.

It is by the same processes that we get most of our explanations of the world as we go through it, and most of the facts on which we base judgment and action. When the same sort of thing happens in a number of fairly different cases, we begin to suspect that there is a reason; and if we are going to make an argument on the subject, we take note of the cases and try in some way to arrange and tabulate them. The supporters of a protective tariff collect instances of prosperity under such a tariff, the supporters of free trade cases of prosperity under free trade, the believers in the classical education cases of men trained in that way who have attained to eminence, believers in the elective system cases of men who are the products of that system who have attained equal eminence. In most cases such collection of instances does not carry you far toward a coercive argument; the cases are too complex for you to assert that any one factor is the cause of the result.

In another kind of case you can come a little nearer. In an argument for the establishment of a commission form of government in a given city or town there are now enough cases of this type of government in practice to make possible a good argument by the method of agreement; the places are scattered over the country, north and south, east and west, and range greatly in size and environments; and all of them so far (1911) report improvement in efficiency and honesty of government. Accordingly it is a fair presumption that the improvement is due to the introduction of the new form of government, since in all other respects the places which have tried it have little in common.

A more important result of the inquiry is to lead us on to an application of the method of difference. Starting with this strong probability that the improvement is due to the new form of government, we can go a step further and examine a single case, in order to establish more clearly the sequence of events which we call a cause. In the case of any given town which has adopted the commission government the material for the application of the method of difference is ready to our hands, if nothing else has been changed in the town but the form of government. The inhabitants and the voters are the same, the physical conditions are the same. If now we seek for the cause of an admitted improvement in the administration of the city affairs, we are driven to ascribe it to the only factor in the case which has been changed, and this is the form of government. Such an argument, if supported by figures and specific facts, is obviously strong.

The same kind of argument is constantly used in the discussion of prohibition and local option as a means of reducing the amount of liquor consumed in a community, for the frequent changes both in states and in smaller communities provide material for the application of the same method of difference. Here, however, the factors are more complex, on account of differences in the character of the population in different places, and their inherited habits as concerns the use of wine, beer, and other liquors.

40. Faulty Generalization. Both generalization through the method of agreement, and the assignment of causes through the method of difference, however, have their dangers, like all forms of reasoning. A discussion of these dangers will throw light on the processes themselves.

The chief danger when you reason through the method of agreement is of jumping to a conclusion too soon, and before you have collected enough cases for a safe conclusion. This is to commit the fallacy known as hasty generalization. It is the error committed by the dogmatic sort of globetrotter, who after six weeks spent in Swiss-managed hotels in Italy will supply you with a full set of opinions on the government, morals, and customs of the country. In a less crass form it affects the judgment of most Englishmen who write books about this country, for they come over with letters of introduction to New York, Boston, Chicago, and San Francisco, and then generalize about the rest of the country and its population.

We are all in danger from the fallacy, however, for it is a necessary law of the mind that we shall begin to make opinions and judgments on a subject as soon as we become acquainted with it. The only safeguards are, in the first place, to keep these preliminary judgments tentative and fluid, and in the second, to keep them to one's self until there is some need of expressing them. The path to wisdom in action is through open-mindedness and caution.

When one has to refute an argument in which there is faulty generalization, it is often easy to point out that its author had no sufficient time or chance to make observations, or to point out that the instances on which he relied are not fair examples of their class. In practice the strength of an argument in which this error is to be found lies largely in the positiveness with which it is pronounced; for it is human nature to accept opinions which have an outward appearance of certainty.

A not uncommon form of faulty generalization is to base an argument on a mere enumeration of similar cases. This is a poor foundation for an argument, especially for a probability in the future, unless the enumeration approaches an exhaustive list of all possible cases. To have reasoned a few years ago that because Yale had beaten Harvard at rowing almost every year for fifteen years it had a permanent superiority in the strength and skill of its oarsmen would have been dangerous, for when the years before the given period were looked up they would have shown results the other way. And an enumeration may run through a very long period of time, and still in the end be upset.

To an inhabitant of Central Africa fifty years ago, no fact probably appeared to rest on more uniform experience than this, that all human beings are black. To Europeans not many years ago, the proposition, 'All swans are white,' appeared an equally unequivocal instance of uniformity in the course of nature. Further experience has proved to both that they were mistaken; but they had to wait fifty centuries for this experience. During that long time, mankind believed in an uniformity of the course of nature where no such uniformity really existed.[37]

Unless you have so wide and complete a view of your subject that you can practically insure your enumeration as exhaustive, it is not safe to reason that because a thing has always happened so in the past, it will always happen so in the future. The notorious difficulty of proving a negative goes back to this principle.

So closely like hasty generalization that it cannot be clearly separated from it is faulty reasoning that arises from neglecting exceptions to a general principle. All our generalizations, except those that are so near truisms as to be barren of interest, are more or less rough and ready, and the process of refining them is a process of finding exceptions and restating the principle so that it will meet the case of the exceptions.

Darwin is said to have had "the power of never letting exceptions pass unnoticed. Every one notices a fact as an exception when it is striking or frequent, but he had a special instinct for arresting an exception."[38] It was this instinct which made him so cautious and therefore so sure in the statement of his hypotheses: after the idea of natural selection as an explanation of the origin of the species of the natural world had occurred to him, he spent twenty years collecting further facts and verifying observations to test the theory before he gave it to the world. A generalization that the republican form of government produces greater peace and prosperity than the monarchical would neglect the obvious exceptions in the Central American republics; and to make it at all tenable the generalization would have to have some such proviso as, "among peoples of Germanic race." Even then the exceptions would be more numerous than the cases which would fall within the rule.[39] One must cultivate respect for facts in making theories: a theory should always be held so tentatively that any new or unnoticed facts can have their due influence in altering it.

Of the errors in reasoning about a cause none is more common than that known by the older logic as post hoc, ergo propter hoc (after this, therefore on account of it), or more briefly, the post hoc fallacy. All of us who have a pet remedy for a cold probably commit this fallacy two times out of three when we declare that our quinine or rhinitis or camphor pill has cured us; for as a wise old doctor of two generations ago declared, and as the new doctrines of medical research are making clear, in nine cases out of ten nature cures.

Of the same character are the common superstitions of daily life, for example, that if thirteen sit at table together one will die within the year, or that crossing a funeral procession brings misfortune. Where such superstitions are more than playfully held, they are gross cases of calling that a cause which has no relation to the event. Here is another example, from a letter to The Nation:[40]

In the last volume of the Shakespeare controversy, the argument presented "To the Reader" seems fairly to be summarized as follows: The plays are recognized as wonderful; scholars are amazed at the knowledge of the classes in them, lawyers at the law, travelers at the minute accuracy of the descriptions of foreign cities; they show a keen critic of court etiquette and French soldiery; the only possible man of the time with this encyclopedic outlook was Francis Bacon. Both in the original and in the summary there seems a casual connection implied, namely, that the plays are wonderful because of the knowledge, and because of the knowledge Bacon is the author. But, stated thus baldly, the fallacy is obvious. It is not because the author "had by study obtained nearly all the learning that could be gained from books" that the Elizabethans went to see the plays, or that we to-day read them; but it is because there is to be found in them wonderful characterization expressed dramatically, namely, before an audience. And this audience is what the scholars seem to forget. For by it is the dramatist limited, since profundity of thought or skill in allusion is good or bad, artistically, exactly in proportion as the thought is comprehended or the allusion understood.

Sometimes this fallacy is caused by assuming that because a certain result followed an event in the only case known, therefore there was a causal connection. In a hearing before a committee of the Massachusetts legislature on a bill to establish closer relations between Boston and its suburbs, the question was asked of a witness whether he believed that in the case of London "the London police would have been as efficient as they are now if there had been no annexation" of the surrounding towns; he very properly replied: "That's a hard question to answer, because we have only the existing side to look at. We don't know what it would have been as separate communities." Wherever multiple causes are possible for a phenomenon it is unsafe to argue from a single case.

Another form of error in reasoning to a cause is to assume that a fact is simple, when it is really complex, as in the following example:

I do not think I am overstepping the bounds when I say that the headship of no corporation, or state, or even the headship of the United States, requires greater general ability, force of character, or knowledge of administration than the head of administration of a great city like New York or Berlin. The latter we know to be well administered, the former—well, let us say, less so. The whole difference is in the systems. Apply the Berlin system to New York, and you will get Berlin results.

Here the writer wholly ignores all sorts of active causes for this difference: Berlin has a tolerably homogeneous population, New York the most heterogeneous in the world; Germans by nature respect law and authority, and hanker for centralization; Americans make and break laws light-heartedly, and are restive under authority; and one might easily go further.

Arguments that national prosperity has followed a higher or a lower tariff are especially apt to be vitiated by this error. It is not that the tariff has no relation to the prosperity, but that there are other causes intermingled with it which may have had more immediate effect. A bad grain crop or a season of reckless speculation may obliterate all the traceable causes of a change in the tariff. Arguments from motive, too, are apt to fall into this error. It is notorious that human motives are mixed. If you argue that a whole class of business organizations are evil because they have been formed solely for the purpose of making inordinate and oppressive profits, you leave out of sight a motive which is strong among American business men—the interest in seeing a great business more efficiently managed, and the desire to exercise power beneficently; and your argument suffers from its illegitimate assumption of a simple cause. So in the same way if you are arguing for or against the advantages of the elective system in a school or a college, or of a classical education, or of athletics, it would be folly to assume that any one cause or effect covered the whole case. Whenever in an argument you are trying to establish any such large and complex fact, you must be wary lest you thus assume a single cause where in reality there are a legion of causes.

41. Deductive Logic—the Syllogism. Deductive logic, as we have seen, deals with reasoning which passes from general principles to individual cases. Its typical form is the syllogism, in which we pass from two propositions which are given to a third, the conclusion. Of the two former one is a general principle, the other an assertion of a particular case. The classic example of the syllogism, which started with Aristotle and has grown hoary with repetition, and so venerable that it is one of the commonplaces of educated speech, runs as follows: All men are mortal, Socrates is a man, Therefore Socrates is mortal. Here there is the general principle, All men are mortal, and the assertion about the particular case, Socrates is a man. The two have one term in common, men (or more strictly, the class Man), which is known as the middle term, through which we reach the conclusion that the characteristic of mortality in which all men are similar is true also of Socrates, by virtue of his being a man. Of the other terms, mortal, which is the more inclusive, is known as the major term, and Socrates, the less inclusive, as the minor term. The first two propositions are the premises, that which contains the major term being known as the major premise, and the other as the minor premise.

The validity of the syllogism lies, as I have said, in the assertion of a general principle, and the bringing of the particular case in hand under that principle: if the principle is granted as incontrovertible, and the special case as really coming under it, the conclusion is inevitable.

On the syllogism in its various forms deductive logic has built up an imposing structure of rules and conclusions. In practice the value of the syllogism is largely indirect. The trouble with it in itself as a mode of progress in reasoning is twofold: in the first place there are very few general principles which, if you are cautious, you will accept without reservations; and in the second place the crucial question in another set of cases is whether the given case really falls under the general principle. The syllogism, All great statesmen are farsighted, Daniel Webster was a great statesman, Therefore Daniel Webster was farsighted, sounds simple; but two generations have disagreed on the question whether Webster was a great statesman; and both great statesman and farsighted are such vague and inclusive terms that one would either accept a general principle of which they are terms as a harmless truism, or else balk at being asked to grant a proposition which might have unexpected meanings thrust into it. This double difficulty pursues the syllogism as a device for forwarding knowledge: either it sets forth a truth so large and vague that you cannot say whether you accept it for all cases or not, or else the disagreement comes on one of the premises, and unless both the premises are granted, strictly syllogistic reasoning does not get under way.