In hearing a report we are in the position of observers of a series of significant sounds, and we are subject to all the fallacies of observation already mentioned. In an aggravated degree, for words are harder to observe than visible things. Our attention is apt to be more listless than in presence of the actual events. Our minds dwell upon parts of the narrative to the neglect of other parts, and in the coherent story or description that we retain in our memories, sequences are apt to be altered and missing links supplied in accordance with what we were predisposed to hear. Thus hearsay evidence is subject to all the imperfections of the original observer, in addition to the still more insidious imperfections of the second observer.
How quickly in the course of a few such transmissions hearsay loses all evidentiary value is simply illustrated by the game known as Russian Scandal. One of a company, A, writes down a short tale or sketch, and reads it to B. B repeats it to C, C to D, and so on. When it has thus gone the round of the company, the last hearer writes down his version, and it is compared with the original. With every willingness to play fair, the changes are generally considerable and significant.
Sometimes it is possible to compare an oral tradition with a contemporary written record. In one of Mr. Hayward's Essays—"The Pearls and Mock Pearls of History"—there are some examples of this disenchanting process. There is, for instance, a pretty story of an exchange of courtesies between the leaders of the French and English Guards at the battle of Fontenoy. The tradition runs that Lord Charles Hay stepped in front of his men and invited the French Guards to fire, to which M. d'Auteroche with no less chivalry responded: "Monsieur, we never fire first;you fire". What really passed we learn from a letter from Lord Charles Hay to his mother, which happens to have been preserved. "I advanced before our regiment, and drank to the Frenchmen, and told them we were the English Guards, and hoped they would stand till we came, and not swim the Scheldt as they did the Maine at Dettingen." Tradition has changed this lively piece of buffoonery into an act of stately and romantic courtesy. The change was probably made quite unconsciously by some tenth or hundredth transmitter, who remembered only part of the story, and dressed the remainder to suit his own fancy.
The question has been raised, For how long can oral tradition be trusted? Newton was of opinion that it might be trusted for eighty years after the event. Others have named forty years. But if this means that we may believe a story that we find in circulation forty years after the alleged events, it is wildly extravagant. It does injustice to the Mythopœic Faculty of man. The period of time that suffices for the creation of a full-blown myth, must be measured by hours rather than by years. I will give an instance from my own observation, if that has not been entirely discredited by my previous confessions. The bazaars of the East are generally supposed to be the peculiar home of myth, hotbeds in which myths grow with the most amazing speed, but the locality of my myth is Aberdeen. In the summer of 1887 our town set up in one of its steeples a very fine carillon of Belgian bells. There was much public excitement over the event: the descriptions of enthusiastic promoters had prepared us to hear silvery music floating all over the town and filling the whole air. On the day fixed for the inauguration, four hours after the time announced forthe first ceremonial peal, not having heard the bells, I was in a shop and asked if anything had happened to put off the ceremony. "Yes," I was told; "there had been an accident; they had not been properly hung, and when the wife of the Lord Provost had taken hold of a string to give the first pull, the whole machinery had come down." As a matter of fact all that had happened was that the sound of the bells was faint, barely audible a hundred yards from the belfry, and not at all like what had been expected. There were hundreds of people in the streets, and the myth had originated somehow among those who had not heard what they went out to hear. The shop where it was repeated circumstantially to me was in the main street, not more than a quarter of a mile from where the carillon had been played in the hearing of a large but disappointed crowd. I could not help reflecting that if I had been a mediæval chronicler, I should have gone home and recorded the story, which continued to circulate for some days in spite of the newspapers: and two hundred years hence no historian would have ventured to challenge the truth of the contemporary evidence.
It is obvious that the tests applied to descriptive testimony in Courts of Law cannot be applied to the assertions of History. It is a supreme canon of historical evidence that only the statements of contemporaries can be admitted: but most even of their statements must rest on hearsay, and even when the historian professes to have been an eye-witness, the range of his observation is necessarily limited, and hecannot be put into the witness-box and cross-examined. Is there then no way of ascertaining historical fact? Must we reject history as altogether unworthy of credit?
The rational conclusion only is that very few facts can be established by descriptive testimony such as would satisfy a Court of Law. Those who look for such ascertainment are on a wrong track, and are doomed to disappointment. It is told of Sir Walter Raleigh that when he was writing his History of the World, he heard from his prison in the Tower a quarrel outside, tried to find out the rights and the wrongs and the course of it, and failing to satisfy himself after careful inquiry, asked in despair how he could pretend to write the history of the world when he could not find out the truth about what occurred under his own windows. But this was really to set up an impossible standard of historical evidence.
The method of testing historical evidence follows rather the lines of the Newtonian method of Explanation, which we shall afterwards describe. We must treat any historical record as being itself in the first place a fact to be explained. The statement at least is extant: our first question is, What is the most rational way of accounting for it? Can it be accounted for most probably by supposing the event stated to have really occurred with all the circumstances alleged? Or is it a more probable hypothesis that it was the result of an illusion of memory on the part of the original observer, if it professes to be the record of an eye-witness, or on the part of some intermediate transmitter, if it is the record of a tradition? To qualify ourselves to answer the latter kind of question with reasonable probability we must acquaint ourselveswith the various tendencies to error in personal observation and in tradition, and examine how far any of them are likely to have operated in the given case. We must study the operation of these tendencies within our experience, and apply the knowledge thus gained. We must learn from actual observation of facts what the Mythopœic Faculty is capable of in the way of creation and transmutation, and what feats are beyond its powers, and then determine with as near a probability as we can how far it has been active in the particular case before us.
Footnote 1:The Invasion of the Crimea, iii. 124
Footnote 2:The truth is, that we see much less than is commonly supposed. Not every impression is attended to that is made on the retina, and unless we do attend we cannot, properly speaking, be said to see. Walking across to college one day, I was startled by seeing on the face of a clock in my way that it was ten minutes to twelve, whereas I generally passed that spot about twenty minutes to twelve. I hurried on, fearing to be late, and on my arrival found myself in very good time. On my way back, passing the clock again, I looked up to see how much it was fast. It marked ten minutes to eight. It had stopped at that time. When I passed before I had really seen only the minute hand. The whole dial must have been on my retina, but I had looked at or attended to only what I was in doubt about, taking the hour for granted. I am bound to add that my business friends hint that it is only absorbed students that are capable of such mistakes, and that alert men of business are more circumspect. That can only be because they are more alive to the danger of error.
One of the chief contributions of the Old Logic to Inductive Method was a name for a whole important class of misobservations. The fallacy entitledPost Hoc ergo Propter Hoc—"After, therefore, Because of"—consisted in alleging mere sequence as a proof of consequence or causal sequence. The sophist appeals to experience, to observed facts: the sequence which he alleges has been observed. But the appeal is fallacious: the observation on which he relies amounts only to this, that the one event has followed upon the other. This much must be observable in all cases of causal sequence, but it is not enough for proof.Post hoc ergo propter hocmay be taken as a generic name for imperfect proof of causation from observed facts of succession.
The standard example of the fallacy is the old Kentish peasant's argument that Tenterden Steeple was the cause of Goodwin Sands. Sir Thomas More (as Latimer tells the story in one of his Sermons to ridicule incautious inference) had been sent down into Kent as a commissioner to inquire into the cause of the silting up of Sandwich Haven. Among those who came to his court was the oldest inhabitant, andthinking that he from his great age must at least have seen more than anybody else, More asked him what he had to say as to the cause of the sands. "Forsooth, sir," was the greybeard's answer, "I am an old man: I think that Tenterden Steeple is the cause of Goodwin Sands. For I am an old man, and I may remember the building of Tenterden Steeple, and I may remember when there was no steeple at all there. And before that Tenterden Steeple was in building, there was no manner of speaking of any flats or sands that stopped the haven; and, therefore, I think that Tenterden Steeple is the cause of the destroying and decaying of Sandwich Haven."
This must be taken as Latimer meant it to be, as a ridiculous example of a purely imbecile argument from observation, but the appeal to experience may have more show of reason and yet be equally fallacious. The believers in Kenelm Digby's "Ointment of Honour" appealed to experience in support of its efficacy. The treatment was to apply the ointment, not to the wound, but to the sword that had inflicted it, to dress this carefully at regular intervals, and, meantime, having bound up the wound, to leave it alone for seven days. It was observed that many cures followed upon this treatment. But those who inferred that the cure was due to the bandaging of the sword, failed to observe that there was another circumstance that might have been instrumental, namely, the exclusion of the air and the leaving of the wound undisturbed while the natural healing processes went on. And it was found upon further observation that binding up the wound alone answered the purpose equally well whether the sword was dressed or not.
In cases wherepost hocis mistaken forpropter hoc,simple sequence for causal sequence, there is commonly some bias of prejudice or custom which fixes observation on some one antecedent and diverts attention from other circumstances and from what may be observed to follow in other cases. In the minds of Digby and his followers there was probably a veneration for the sword as the weapon of honour, and a superstitious belief in some secret sympathy between the sword and its owner. So when the practice of poisoning was common, and suspicion was flurried by panic fear, observation was often at fault. Pope Clement VIII. was said to have been killed by the fumes of a poisoned candle which was placed in his bedroom. Undoubtedly candles were there, but those who attributed the Pope's death to them took no notice of the fact that a brazier of burning charcoal was at the same time in the apartment with no sufficient outlet for its fumes. Prince Eugene is said to have received a poisoned letter, which he suspected and immediately threw from him. To ascertain whether his suspicions were well founded the letter was administered to a dog, which, to make assurance doubly sure, was fortified by an antidote. The dog died, but no inquiry seems to have been made into the character of the antidote.
Hotspur's retort to Glendower showed a sound sense of the true value to be attached to mere priority.
Glendower.At my nativityThe front of heaven was full of fiery shapes,Of burning cressets: and at my birthThe frame and huge foundation of the earthShaked like a coward.Hotspur. Why so it would have done at the same season, ifyour mother's cat had but kittened, though yourself had neverbeen born.1 Hen. IV., 3, 1, 13.
Glendower.At my nativityThe front of heaven was full of fiery shapes,Of burning cressets: and at my birthThe frame and huge foundation of the earthShaked like a coward.
Glendower.
At my nativity
The front of heaven was full of fiery shapes,
Of burning cressets: and at my birth
The frame and huge foundation of the earth
Shaked like a coward.
Hotspur. Why so it would have done at the same season, ifyour mother's cat had but kittened, though yourself had neverbeen born.
Hotspur. Why so it would have done at the same season, if
your mother's cat had but kittened, though yourself had never
been born.
1 Hen. IV., 3, 1, 13.
1 Hen. IV., 3, 1, 13.
We all admit at once that the retort was just. What principle of sound conclusion was involved in it? It is the business of Inductive Logic to make such principles explicit.
TakingPost Hoc ergo Propter Hocas a generic name for fallacious arguments of causation based on observed facts, for the fallacious proof of causation from experience, the question for Logic is, What more than meresequenceis required to proveconsequence?When do observations ofPost Hocwarrant the conclusionPropter Hoc?
The methods formulated by Mill under the name of Experimental Methods are methods actually practised by men of science with satisfactory results, and are perfectly sound in principle. They were, indeed, in substance, taken by him from the practice of the scientific laboratory and study as generalised by Herschel. In effect what Mill did was to restate them and fit them into a system. But the controversies into which he was tempted in so doing have somewhat obscured their exact function in scientific inquiry. Hostile critics, finding that they did not serve the ends that he seemed to claim for them, have jumped to the conclusion that they are altogether illusory and serve no purpose at all.
First, we must dismiss the notion, encouraged by Mill's general theory of Inference, that the Experimental Methods have anything special to do with the observation and inferential extension of uniformities such as that death is common to all organised beings.One of the Methods, as we shall see, that named by Mill the Method of Agreement, does incidentally and collaterally establish empirical laws in the course of its observations, and this probably accounts for the prominence given to it in Mill's system. But this is not its end and aim, and the leading Method, that named by him the Method of Difference, establishes as fact only a particular case of causal coincidence. It is with the proof of theories of causation that the Experimental Methods are concerned: they are methods of observing with a view to such proof.1
The next point to be made clear is that the facts of causation with which the Methods are concerned are observable facts, relations among phenomena, but that the causal relations or conditions of which they are the proof are not phenomena, in the meaning of being manifest to the senses, but rather noumena, inasmuch as they are reached by reasoning from what is manifest.
Take, for example, what is known as thequaquaversusprinciple in Hydrostatics, that pressure upon a liquid is propagated equally in all directions. We cannot observe this extension of pressure among the liquid particles directly. It cannot be traced among the particles by any of our senses. But we can assume that it is so, consider what ought to be visible if it is so, and then observe whether the visible facts are in accordance with the hypothesis. A box can be made, filled with water, and so fitted with pistons on top and bottom and on each of its four sides that they willindicate the amount of pressure on them from within. Let pressure then be applied through a hole in the top, and the pistons show that it has been communicated to them equally. The application of the pressure and the yielding of the pistons are observable facts, facts in causal sequence: what happens among the particles of the liquid is not observed but reasonably conjectured, is notphenomenalbutnoumenal.
This distinction, necessary to an understanding of the scope of the Methods, was somewhat obscured by Mill in his preliminary discussion of the meaning of "cause". Very rightly, though somewhat inconsistently with his first theory of Induction, he insists that "the notion of Cause being the root of the whole theory of Induction, it is indispensable that this idea should at the very outset of our inquiry be, with the utmost practicable degree of precision, fixed and determined". But in this determination, not content with simply recognising that it is with phenomena that the Experimental Methods primarily deal, it being indeed only phenomena that can be the subjects of experimental management and observation, he starts by declaring that science has not to do with any causes except such as are phenomenal—"when I speak of the cause of any phenomenon, I do not mean a cause which is not itself a phenomenon"—and goes on to define as the only correct meaning of cause "the sum total of conditions," including among them conditions which are not phenomenal, in the sense of being directly open to observation.
When Mill protested that he had regard only to phenomenal causes, he spoke as the partisan of a philosophical tradition. It would have been well if he had acted upon his own remark that the properunderstanding of the scientific method of investigating cause is independent of metaphysical analysis of what cause means. Curiously enough, this remark is the preface to an analysis of cause which has but slight relevance to science, and is really the continuation of a dispute begun by Hume. This is the key to his use of the word phenomenon: it must be interpreted with reference to this: when he spoke of causes as phenomenal, he opposed the word to "occult" in some supposed metaphysical sense.2And this irrelevant discussion, into the vortex of which he allowed himself to be carried, obscured the fact, elsewhere fully recognised by Mill himself, that science does attempt to get beyond phenomena at ultimate laws which are not themselves phenomena though they bind phenomena together. The "colligation" of the facts, to use Whewell's phrase, is not a phenomenon, but a noumenon.
The truth is that a very simple analysis of "cause" is sufficient for the purposes of scientific inquiry. It is enough to make sure that causal sequence or consequence shall not be confounded with simple sequence. Causal sequence is simple sequence and something more, that something more being expressed by calling it causal. What we call a cause is not merely antecedent or prior in time to what we call its effect: it is so related to the effect that if it or an equivalent event had not happened the effect would not have happened. Anything in the absence of which aphenomenon would not have come to pass as it did come to pass is a cause in the ordinary sense. We may describe it as an indispensable antecedent, with this reservation (which will be more fully understood afterwards), that if we speak of a general effect, such as death, the antecedents must be taken with corresponding generality.
It is misleading to suggest, as Mill does, by defining cause as "the sum total of conditions"—a definition given to back up his conception of cause as phenomenal—that science uses the word cause in a different meaning from that of ordinary speech. It is quite true that "the cause, philosophically speaking, is the sum total of the conditions, positive and negative, taken together: the whole of the contingencies of every description, which being realised, the consequent invariably follows". But this does not imply any discrepancy between the scientific or philosophical meaning and the ordinary meaning. It is only another way of saying that the business of science or philosophy is to furnish a complete explanation of an event, an account of all its indispensable antecedents. The plain man would not refuse the name of cause to anything that science or philosophy could prove to be an indispensable antecedent, but his interest in explanation is more limited. It is confined to what he wants to know for the purpose he has in hand. Nor could the man of science consistently refuse the name of cause to what the plain man applies it to, if it really was something in consequence of which the event took place. Only his interest in explanation is different. The indispensable antecedents that he wants to know may not be the same. Science or philosophy applies itself to the satisfaction of a widercuriosity: it wants to know all the causes, the whole why, the sum total of conditions. To that end the various departments of science interest themselves in various species of conditions. But all understand the word cause in the ordinary sense.
We must not conclude from accidental differences in explanation or statement of cause, dependent on the purpose in view, that the word Cause is used in different senses. In answering a question as to the cause of anything, we limit ourselves to what we suppose our interrogator to be ignorant of and desirous of knowing. If asked why the bells are ringing, we mention a royal marriage, or a victory, or a church meeting, or a factory dinner hour, or whatever the occasion may be. We do not consider it necessary to mention that the bells are struck by a clapper. Our hearer understands this without our mentioning it. Nor do we consider it necessary to mention the acoustic condition, that the vibration of the bells is communicated to our ears through the air, or the physiological condition, that the vibrations in the drums of our ears are conveyed by a certain mechanism of bone and tissue to the nerves. Our hearer may not care to know this, though quite prepared to admit that these conditions are indispensable antecedents. Similarly, a physiographer, in stating the cause of the periodical inundation of the Nile, would consider it enough to mention the melting of snow on the mountains in the interior of Africa, without saying anything of such conditions as the laws of gravity or the laws of liquefaction by heat, though he knows that these conditions are also indispensable. Death is explained by the doctor when referred to a gunshot wound, or a poison, or a virulent disease. The Pathologist mayinquire further, and the Moral Philosopher further still. But all inquiries into indispensable conditions are inquiries into cause. And all alike have to be on their guard against mistaking simple sequence for consequence.
To speak of the sum total of conditions, as the Cause in a distinctively scientific sense, is misleading in another direction. It rather encourages the idea that science investigates conditions in the lump, merely observing the visible relations between sets of antecedents and their consequents. Now this is the very thing that science must avoid in order to make progress. It analyses the antecedent situation, tries to separate the various coefficients, and finds out what they are capable of singly. It must recognise that some of the antecedents of which it is in search are not open to observation. It is these, indeed, for the most part that constitute the special subject-matter of the sciences in Molar as well as in Molecular Physics. For practical every-day purposes, it is chiefly the visible succession of phenomena that concerns us, and we are interested in the latent conditions only in as far as they provide safer ground for inference regarding such visible succession. But to reach the latent conditions is the main work of science.
It is, however, only through observation of what is open to the senses that science can reach the underlying conditions, and, therefore, to understand its methods we must consider generally what is open to observation in causal succession. What can be observed when phenomena follow one another as cause and effect, that is, when the one happens in consequence of the happening of the other? In Hume's theory, which Mill formally adopted with a modification,3there is nothing observable but the constancy or invariability of the connexion. When we say that Fire burns, there is nothing to be observed except that a certain sensation invariably follows upon close proximity to fire. But this holds good only if our observation is arbitrarily limited to the facts enounced in the expression. If this theory were sound, science would be confined to the observation of empirical laws. But that there is something wrong with it becomes apparent when we reflect that it has been ascertained beyond doubt that in many observed changes, and presumably in all, there is a transference of energy from one form to another. The paralogism really lies in the assumption from which Hume deduced his theory, namely, that every idea is a copy of some impression. As a matter of fact, we have ideas that are not copies of any one impression, but a binding together, colligation, or intellection of several impressions. Psychological analysis shows us that even when we say that things exist with certain qualities, we are expressing not single impressions or mental phenomena, but supposed causes and conditions of such,noumenain short, which connect our recollections of many separate impressions and expectations of more.
The Experimental Methods proceed on the assumption that there is other outward and visible evidenceof causal connexion than invariability of sequence. In the leading Method it is assumed that when events may be observed to follow one another in a certain way, they are in causal sequence. If we can make sure that an antecedent change is the only change that has occurred in an antecedent situation, we have proof positive that any immediately subsequent change in the situation is a consequent, that the successive changes are in causal sequence. Thus when Pascal's barometer was carried to the top of Puy le Dome, and the mercury in it fell, the experimenters argued that the fall of the mercury was causally connected with the change of elevation, all the other circumstances remaining the same. This is the foundation of the so-called Method of Difference. To determine that the latent condition was a difference in the weight of the atmosphere, needed other observations, calculations and inferences; but if it could be shown that the elevation was the only antecedent changed in a single instance, causal connexion was established between this and the phenomenon of the fall of the barometer.
It is obvious that in coming to this conclusion we assume what cannot be demonstrated but must simply be taken as a working principle to be confirmed by its accordance with experience, that nothing comes into being without some change in the antecedent circumstances. This is the assumption known as the Law of Causation—ex nihilo nihil fit.
Again, certain observable facts are taken as evidence that there is no causal connexion. On the assumption that any antecedent in whose absence a phenomenon takes place is not causally connected with it, we set aside or eliminate various antecedents as fortuitous or non-causal. This negative principle, as we shall see,is the foundation of what Mill called the Method of Agreement.
Be it remarked, once for all, that before coming to a conclusion on the Positive Method or Method of Difference, we may often have to make many observations on the Negative Method. Thus Pascal's experimenters, before concluding that the change of altitude was the only influential change, tried the barometer in exposed positions and in sheltered, when the wind blew and when it was calm, in rain and in fog, in order to prove that these circumstances were indifferent. We must expound and illustrate the methods separately, but every method known to science may have in practice to be employed in arriving at a single conclusion.
Footnote 1:This is implied, as I have already remarked, in the word Experimental. An experiment is a proof or trial: of what? Of a theory, a conjecture.
Footnote 2:If we remember, as becomes apparent on exact psychological analysis, that things and their qualities are as muchnoumenaand not, strictly speaking,phenomenaas the attraction of gravity or the quaquaversus principle in liquid pressure, the prejudice against occultism is mitigated.
Footnote 3:The modification was that causation is not only "invariable" but also "unconditional" sequence. This addition of unconditionality as part of the meaning of cause, after defining cause as the sum total of the conditions, is very much like arguing in a circle. After all, the only point recognised in the theory as observable is the invariability of the sequence. But this is less important than the fact that in his canons of the Experimental Methods Mill recognised that more is observable.
On what principle do we decide, in watching a succession of phenomena, that they are connected as cause and effect, that one happened in consequence of the happening of another? It may be worded as follows:—
When the addition of an agent is followed by the appearance or its subtraction by the disappearance of a certain effect, no other influential circumstance having been added or subtracted at the same time or in the meantime, and no change having occurred among the original circumstances, that agent is a cause of the effect.
When the addition of an agent is followed by the appearance or its subtraction by the disappearance of a certain effect, no other influential circumstance having been added or subtracted at the same time or in the meantime, and no change having occurred among the original circumstances, that agent is a cause of the effect.
On this principle we would justify our belief in the causal properties of common things—that fire burns, that food appeases hunger, that water quenches thirst, that a spark ignites gunpowder, that taking off a tight shoe relieves a pinched foot. We have observed the effect following when there was no other change in the antecedent circumstances, when the circumstance to which we refer it was simply added to or subtracted from the prior situation.
Suppose we doubt whether a given agent is or is not capable of producing a certain effect in certain circumstances, how do we put it to the proof? We add it singly or subtract it singly, taking care that everything else remains as before, and watch the result. If we wish to know whether a spoonful of sugar can sweeten a cup of tea, we taste the tea without the sugar, then add the sugar, and taste again. The isolated introduction of the agent is the proof, the experiment. If we wish to know whether a pain in the foot is due to a tight lacing, we relax the lacing and make no other change: if the pain then disappears, we refer it to the lacing as the cause. The proof is the disappearance of the pain on the subtraction of the single antecedent.
The principle on which we decide that there is causal connexion is the same whether we make the experimental changes ourselves or merely watch them as they occur—the only course open to us with the great forces of nature which are beyond the power of human manipulation. In any case we have proof of causation when we can make sure that there was only one difference in the antecedent circumstances corresponding to the difference of result.
Mill's statement of this principle, which he calls the Canon of the Method of Difference, is somewhat more abstract, but the proof relied upon is substantially the same.
If an instance in which the phenomenon under investigation 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]1the cause, or an indispensable part of the cause, of the phenomenon.
If an instance in which the phenomenon under investigation 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]1the cause, or an indispensable part of the cause, of the phenomenon.
Mill's statement has the merit of exactness, but besides being too abstract to be easy of application, the canon is apt to mislead in one respect. The wording of it suggests that the two instances required must be two separate sets of circumstances, such as may be put side by side and compared, one exhibiting the phenomenon and the other not. Now in practice it is commonly one set of circumstances that we observe with a special circumstance introduced or withdrawn: the two instances, the data of observation, are furnished by the scene before and the scene after the experimental interference. In the case, for example, of a man shot in the head and falling dead, death being the phenomenon in question, the instance where it does not occur is the man's condition before he received the wound, and the instance where it does occur is his condition after, the single circumstance of difference being the wound, a difference produced by the addition or introduction of a new circumstance. Again, take the common coin and feather experiment, contrived to show that the resistance of the air is the cause of thefeather's falling to the ground more slowly than the coin. The phenomenon under investigation is the retardation of the feather. When the two are dropped simultaneously in the receiver of an air-pump, the air being left in, the feather flutters to the ground after the coin. This is the instance where the phenomenon occurs. Then the air is pumped out of the receiver, and the coin and the feather being dropped at the same instant reach the ground together. This is the instance where the phenomenon does not occur. The single circumstances of difference is the presence of air in the former instance, a difference produced by the subtraction of a circumstance.
Mill's Canon is framed so as to suit equally whether the significant difference is produced by addition to or subtraction from an existing sum of circumstances. But that is misleading in so far as it suggests that the two instances must be separate sets of circumstances, is shown by the fact that it misled himself when he spoke of the application of the method in social investigations, such as the effect of Protection on national wealth. "In order," he says, "to apply to the case the most perfect of the methods of experimental inquiry, the Method of Difference, we require to find two instances which tally in every particular except the one which is the subject of inquiry. We must have two nations alike in all natural advantages and disadvantages; resembling each other in every quality physical and moral; habits, usages, laws, and institutions, and differing only in the circumstance that the one has a prohibitory tariff and the other has not." It being impossible ever to find two such instances, he concluded that the Method of Difference could not be applied in social inquiries. But really it is not necessaryin order to have two instances that we should have two different nations: the same nation before and after a new law or institution fulfils that requirement. The real difficulty, as we shall see, is to satisfy the paramount condition that the two instances shall differ in a single circumstance. Every new enactment would be an experiment after the Method of Difference, if all circumstances but it remained the same till its results appeared. It is because this seldom or never occurs that decisive observation is difficult or impossible, and the simple method of difference has to be supplemented by other means.
To introduce or remove a circumstance singly is the typical application of the principle; but it may be employed also to compare the effects of different agents, each added alone to exactly similar circumstances. A simple example is seen in Mr. Jamieson's agricultural experiments to determine the effects of different manures, such as coprolite and superphosphate, on the growth of crops. Care is taken to have all the antecedent circumstances as exactly alike as possible, except as regards the agency whose effects are to be observed. A field is chosen of uniform soil and even exposure and divided into plots: it is equally drained so as to have the same degree of moisture throughout; the seed is carefully selected for the whole sowing. Between the sowing and the maturing of the crop all parts of the field are open to the same weather. Each plot may thus be regarded as practically composing the same set of conditions, and any difference in the product may with reasonable probability be ascribed to the single difference in the antecedents, the manures which it is desired to compare.
The principle of referring a phenomenon to the only immediately preceding change in antecedent circumstances that could possibly have affected it, is so simple and so often employed by everybody every day, that at first we do not see how there can be any difficulty about it or any possibility of error. And once we understand how many difficulties there are in reaching exact knowledge even on this simple principle, and what care has to be taken, we are apt to overrate its value, and to imagine that it carries us further than it really does. The scientific expert must know how to apply this principle, and a single application of it with the proper precautions may take him days or weeks, and yet all that can be made good by it may carry but a little way towards the knowledge of which he is in search.
When the circumstances are simple and the effect follows at once, as when hot water scalds, or a blow with a stick breaks a pane of glass, there can be no doubt of the causal connexion so far, though plenty of room for further inquiry into the why. But the mere succession of phenomena may be obscure. We may introduce more than one agent without knowing it, and if some time elapses between the experimental interference and the appearance of the effect, other agents may come in without our knowledge.
We must know exactly what it is that we introduce and all the circumstances into which we introduce it. We are apt to ignore the presence of antecedents that are really influential in the result. A man heated by work in the harvest field hastily swallows a glass of water, and drops down dead. There is no doubt thatthe drinking of the water was a causal antecedent, but the influential circumstance may not have been the quantity or the quality of the liquid but its temperature, and this was introduced into the situation as well as a certain amount of the liquid components. In making tea we put in so much tea and so much boiling water. But the temperature of the pot is also an influential circumstance in the resulting infusion. So in chemical experiments, where one might expect the result to depend only upon the proportions of the ingredients, it is found that the quantity is also influential, the degree of heat evolved entering as a factor into the result. Before we can apply the principle of single difference, we must make sure that there is really only a single difference between the instances that we bring into comparison.
The air-pump was invented shortly before the foundation of the Royal Society, and its members made many experiments with this new means of isolating an agent and thus discovering its potentialities. For example, live animals were put into the receiver, and the air exhausted, with the result that they quickly died. The absence of the air being the sole difference, it was thus proved to be indispensable to life. But air is a composite agent, and when means were contrived of separating its components, the effects of oxygen alone and of carbonic acid alone were experimentally determined.
A good example of the difficulty of excluding agencies other than those we are observing, of making sure that none such intrude, is found in the experiments that have been made in connexion with spontaneous generation. The question to be decided is whether life ever comes into existence without the antecedentpresence of living germs. And the method of determining this is to exclude all germs rigorously from a compound of inorganic matter, and observe whether life ever appears. If we could make sure in any one case that no germs were antecedently present, we should have proved that in that case at least life was spontaneously generated.
The difficulty here arises from the subtlety of the agent under observation. The notion that maggots are spontaneously generated in putrid meat, was comparatively easy to explode. It was found that when flies were excluded by fine wire-gauze, the maggots did not appear. But in the case of microscopic organisms proof is not so easy. The germs are invisible, and it is difficult to make certain of their exclusion. A French experimenter, Pouchet, thought he had obtained indubitable cases of spontaneous generation. He took infusions of vegetable matter, boiled them to a pitch sufficient to destroy all germs of life, and hermetically sealed up the liquid in glass flasks. After an interval, micro-organisms appeared. Doubts as to the conclusion that they had been spontaneously generated turned upon two questions: whether all germs in the liquid had been destroyed by the preliminary boiling, and whether germs could have found access in the course of the interval before life appeared. At a certain stage in Pouchet's process he had occasion to dip the mouths of the flasks in mercury. It occurred to Pasteur in repeating the experiments that germs might have found their way in from the atmospheric dust on the surface of this mercury. That this was so was rendered probable by his finding that when he carefully cleansed the surface of the mercury no life appeared afterwards in his flasks.
The application of the principle in human affairs is rendered uncertain by the immense complication of the phenomena, the difficulty of experiment, and the special liability of our judgments to prejudice. That men and communities of men are influenced by circumstances is not to be denied, and the influence of circumstances, if it is to be traced at all, must be traced through observed facts. Observation of the succession of phenomena must be part at least of any method of tracing cause and effect. We must watch what follows upon the addition of new agencies to a previously existing sum. But we can seldom or never get a decisive observation from one pair of instances, a clear case of difference of result preceded by a single difference in the antecedents. The simple Method of Experimental Addition or Subtraction is practically inapplicable. We can do nothing with a man analogous to putting him into a hermetically sealed retort. Any man or any community that is the subject of our observations must be under manifold influences. Each of them probably works some fraction of the total change observable, but how are they to be disentangled? Consider, for example, how impossible it would be to prove in an individual case, on the strict principle of Single Difference, that Evil communications corrupt good manners. Moral deterioration may be observed following upon the introduction of an evil companion, but how can we make sure that no other degrading influence has operated, and that no original depravity has developed itself in the interval? Yet such propositions of moral causation can be proved from experience with reasonable probability. Only it must be by more extended observations than the strict Method of Difference takes into account. The methodis to observe repeated coincidences between evil companionship and moral deterioration, and to account for this in accordance with still wider observations of the interaction of human personalities.
For equally obvious reasons the simple Method of Difference is inapplicable to tracing cause and effect in communities. Every new law or repeal of an old law is the introduction of a new agency, but the effects of it are intermixed with the effects of other agencies that operate at the same time. Thus Professor Cairnes remarks, concerning the introduction of a high Protective Tariff into the United States in 1861, that before its results could appear in the trade and manufacture of the States, there occurred (1) The great Civil War, attended with enormous destruction of capital; (2) Consequent upon this the creation of a huge national debt, and a great increase of taxation; (3) The issue of an inconvertible paper currency, deranging prices and wages; (4) The discovery of great mineral resources and oil-springs; (5) A great extension of railway enterprise. Obviously in such circumstances other methods than the Method of Difference must be brought into play before there can be any satisfactory reasoning on the facts observed. Still what investigators aim at is the isolation of the results of single agencies.
Footnote 1:Prof. Bain, who adopts Mill's Canon, silently drops the words within brackets. They seem to be an inadvertence. The "circumstance," in all the examples that Mill gives, is an antecedent circumstance. Herschel's statement, of which Mill's is an adaptation, runs as follows: "If we can either find produced by nature, or produce designedly for ourselves, two instances which agree exactly in all but one particular and differ in that one, its influence in producing the phenomenon, if it have any, must thereby be rendered apparent".