In illustrating1 these methods, it will be necessary to bear in mind the twofold character of inquiries2 into the laws of phenomena3; which may be either inquiries into the cause of a given effect, or into the effects or properties of a given cause. We shall consider the methods in their application to either order of investigation4, and shall draw our examples equally from both.
We shall denote antecedents by the large letters of the alphabet, and the consequents corresponding to them by the small. Let A, then, be an agent or cause, and let the object of our inquiry5 be to ascertain6 what are the effects of this cause. If we can either find, or produce, the agent A in such varieties of circumstances, that the different cases have no circumstance in common except A; then whatever effect we find to be produced in all our trials, is indicated as the effect of A. Suppose, for example, that A is tried along with B and C, and that the effect is a b c; and suppose that A is next tried with D and E, but without B and C, and that the effect is a d e. Then we may reason thus: b and c are not effects of A, for they were not produced by it in the second experiment; nor [Pg 426]are d and e, for they were not produced in the first. Whatever is really the effect of A must have been produced in both instances; now this condition is fulfilled by no circumstance except a. The phenomenon a cannot have been the effect of B or C, since it was produced where they were not; nor of D or E, since it was produced where they were not. Therefore it is the effect of A.
For example, let the antecedent A be the contact of an alkaline substance and an oil. This combination being tried under several varieties of circumstances, resembling each other in nothing else, the results agree in the production of a greasy7 and detersive or saponaceous substance: it is therefore concluded that the combination of an oil and an alkali causes the production of a soap. It is thus we inquire, by the Method of Agreement, into the effect of a given cause.
In a similar manner we may inquire into the cause of a given effect. Let a be the effect. Here, as shown in the last chapter, we have only the resource of observation without experiment: we cannot take a phenomenon of which we know not the origin, and try to find its mode of production by producing it: if we succeeded in such a random8 trial it could only be by accident. But if we can observe a in two different combinations, a b c, and a d e; and if we know, or can discover, that the antecedent circumstances in these cases respectively were A B C and A D E; we may conclude by a reasoning similar to that in the preceding example, that A is the antecedent connected with the consequent a by a law of causation. B and C, we may say, cannot be causes of a, since on its second occurrence they were not present; nor are D and E, for they were not present on its first occurrence. A, alone of the five circumstances, was found among the antecedents of a in both instances.
For example, let the effect a be crystallization. We compare instances in which bodies are known to assume crystalline structure, but which have no other point of agreement; and we find them to have one, and as far as we can observe, only one, antecedent in common: the deposition9 of a solid matter from a liquid state, either a state of fusion10 or of solution. We conclude, [Pg 427]therefore, that the solidification11 of a substance from a liquid state is an invariable antecedent of its crystallization.
In this example we may go farther, and say, it is not only the invariable antecedent but the cause; or at least the proximate event which completes the cause. For in this case we are able, after detecting the antecedent A, to produce it artificially, and by finding that a follows it, verify the result of our induction12. The importance of thus reversing the proof was strikingly manifested when by keeping a phial of water charged with siliceous particles undisturbed for years, a chemist (I believe Dr. Wollaston) succeeded in obtaining crystals of quartz13: and in the equally interesting experiment in which Sir James Hall produced artificial marble, by the cooling of its materials from fusion under immense pressure: two admirable examples of the light which may be thrown upon the most secret processes of nature by well-contrived interrogation of her.
But if we cannot artificially produce the phenomenon A, the conclusion that it is the cause of a remains15 subject to very considerable doubt. Though an invariable, it may not be the unconditional16 antecedent of a, but may precede it as day precedes night or night day. This uncertainty17 arises from the impossibility of assuring ourselves that A is the only immediate18 antecedent common to both the instances. If we could be certain of having ascertained19 all the invariable antecedents, we might be sure that the unconditional invariable antecedent, or cause, must be found somewhere among them. Unfortunately it is hardly ever possible to ascertain all the antecedents, unless the phenomenon is one which we can produce artificially. Even then, the difficulty is merely lightened, not removed: men knew how to raise water in pumps long before they adverted21 to what was really the operating circumstance in the means they employed, namely, the pressure of the atmosphere on the open surface of the water. It is, however, much easier to analyse completely a set of arrangements made by ourselves, than the whole complex mass of the agencies which nature happens to be exerting at the moment of the production of a given phenomenon. [Pg 428]We may overlook some of the material circumstances in an experiment with an electrical machine; but we shall, at the worst, be better acquainted with them than with those of a thunder-storm.
The mode of discovering and proving laws of nature, which we have now examined, proceeds on the following axiom: Whatever circumstances can be excluded, without prejudice to the phenomenon, or can be absent notwithstanding its presence, is not connected with it in the way of causation. The casual circumstances being thus eliminated, if only one remains, that one is the cause which we are in search of: if more than one, they either are, or contain among them, the cause; and so, mutatis mutandis, of the effect. As this method proceeds by comparing different instances to ascertain in what they agree, I have termed it the Method of Agreement: and we may adopt as its regulating principle the following canon:—
First Canon.
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.
Quitting for the present the Method of Agreement, to which we shall almost immediately return, we proceed to a still more potent22 instrument of the investigation of nature, the Method of Difference.
§ 2. In the Method of Agreement, we endeavoured to obtain instances which agreed in the given circumstance but differed in every other: in the present method we require, on the contrary, two instances resembling one another in every other respect, but differing in the presence or absence of the phenomenon we wish to study. If our object be to discover the effects of an agent A, we must procure23 A in some set of ascertained circumstances, as A B C, and having noted24 the effects produced, compare them with the effect of the remaining circumstances B C, when A is absent. If the effect of A B C is a b c, and the effect of B C, b c, it is [Pg 429]evident that the effect of A is a. So again, if we begin at the other end, and desire to investigate the cause of an effect a, we must select an instance, as a b c, in which the effect occurs, and in which the antecedents were A B C, and we must look out for another instance in which the remaining circumstances, b c, occur without a. If the antecedents, in that instance, are B C, we know that the cause of a must be A: either A alone, or A in conjunction with some of the other circumstances present.
It is scarcely necessary to give examples of a logical process to which we owe almost all the inductive conclusions we draw in daily life. When a man is shot through the heart, it is by this method we know that it was the gun-shot which killed him: for he was in the fulness of life immediately before, all circumstances being the same, except the wound.
The axioms implied in this method are evidently the following. Whatever antecedent cannot be excluded without preventing the phenomenon, is the cause, or a condition, of that phenomenon: Whatever consequent can be excluded, with no other difference in the antecedents than the absence of a particular one, is the effect of that one. Instead of comparing different instances of a phenomenon, to discover in what they agree, this method compares an instance of its occurrence with an instance of its non-occurrence, to discover in what they differ. The canon which is the regulating principle of the Method of Difference may be expressed as follows:
Second Canon.
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 the cause, or an indispensable part of the cause, of the phenomenon.
§ 3. The two methods which we have now stated have many features of resemblance, but there are also many distinctions [Pg 430]between them. Both are methods of elimination25. This term (employed in the theory of equations to denote the process by which one after another of the elements of a question is excluded, and the solution made to depend on the relation between the remaining elements only) is well suited to express the operation, analogous26 to this, which has been understood since the time of Bacon to be the foundation of experimental inquiry: namely, the successive exclusion27 of the various circumstances which are found to accompany a phenomenon in a given instance, in order to ascertain what are those among them which can be absent consistently with the existence of the phenomenon. The Method of Agreement stands on the ground that whatever can be eliminated, is not connected with the phenomenon by any law. The Method of Difference has for its foundation, that whatever cannot be eliminated, is connected with the phenomenon by a law.
Of these methods, that of Difference is more particularly a method of artificial experiment; while that of Agreement is more especially the resource employed where experimentation28 is impossible. A few reflections will prove the fact, and point out the reason of it.
It is inherent in the peculiar29 character of the Method of Difference, that the nature of the combinations which it requires is much more strictly30 defined than in the Method of Agreement. The two instances which are to be compared with one another must be exactly similar, in all circumstances except the one which we are attempting to investigate: they must be in the relation of A B C and B C, or of a b c and b c. It is true that this similarity of circumstances needs not extend to such as are already known to be immaterial to the result. And in the case of most phenomena we learn at once, from the commonest experience, that most of the coexistent phenomena of the universe may be either present or absent without affecting the given phenomenon; or, if present, are present indifferently when the phenomenon does not happen and when it does. Still, even limiting the identity which is required between the two instances, A B C and B C, to such circumstances as are not already known to be indifferent; it is [Pg 431]very seldom that nature affords two instances, of which we can be assured that they stand in this precise relation to one another. In the spontaneous operations of nature there is generally such complication and such obscurity, they are mostly either on so overwhelmingly large or on so inaccessibly31 minute a scale, we are so ignorant of a great part of the facts which really take place, and even those of which we are not ignorant are so multitudinous, and therefore so seldom exactly alike in any two cases, that a spontaneous experiment, of the kind required by the Method of Difference, is commonly not to be found. When, on the contrary, we obtain a phenomenon by an artificial experiment, a pair of instances such as the method requires is obtained almost as a matter of course, provided the process does not last a long time. A certain state of surrounding circumstances existed before we commenced the experiment; this is B C. We then introduce A; say, for instance, by merely bringing an object from another part of the room, before there has been time for any change in the other elements. It is, in short (as M. Comte observes), the very nature of an experiment, to introduce into the pre-existing state of circumstances a change perfectly32 definite. We choose a previous state of things with which we are well acquainted, so that no unforeseen alteration33 in that state is likely to pass unobserved; and into this we introduce, as rapidly as possible, the phenomenon which we wish to study; so that in general we are entitled to feel complete assurance that the pre-existing state, and the state which we have produced, differ in nothing except the presence or absence of that phenomenon. If a bird is taken from a cage, and instantly plunged34 into carbonic acid gas, the experimentalist may be fully35 assured (at all events after one or two repetitions) that no circumstance capable of causing suffocation36 had supervened in the interim37, except the change from immersion38 in the atmosphere to immersion in carbonic acid gas. There is one doubt, indeed, which may remain in some cases of this description; the effect may have been produced not by the change, but by the means employed to produce the change. The possibility, however, of this last supposition generally admits of [Pg 432]being conclusively39 tested by other experiments. It thus appears that in the study of the various kinds of phenomena which we can, by our voluntary agency, modify or control, we can in general satisfy the requisitions of the Method of Difference; but that by the spontaneous operations of nature those requisitions are seldom fulfilled.
The reverse of this is the case with the Method of Agreement. We do not here require instances of so special and determinate a kind. Any instances whatever, in which nature presents us with a phenomenon, may be examined for the purposes of this method; and if all such instances agree in anything, a conclusion of considerable value is already attained40. We can seldom, indeed, be sure that the one point of agreement is the only one; but this ignorance does not, as in the Method of Difference, vitiate the conclusion; the certainty of the result, as far as it goes, is not affected41. We have ascertained one invariable antecedent or consequent, however many other invariable antecedents or consequents may still remain unascertained. If A B C, A D E, A F G, are all equally followed by a, then a is an invariable consequent of A. If a b c, a d e, a f g, all number A among their antecedents, then A is connected as an antecedent, by some invariable law, with a. But to determine whether this invariable antecedent is a cause, or this invariable consequent an effect, we must be able, in addition, to produce the one by means of the other; or, at least, to obtain that which alone constitutes our assurance of having produced anything, namely, an instance in which the effect, a, has come into existence, with no other change in the pre-existing circumstances than the addition of A. And this, if we can do it, is an application of the Method of Difference, not of the Method of Agreement.
It thus appears to be by the Method of Difference alone that we can ever, in the way of direct experience, arrive with certainty at causes. The Method of Agreement leads only to laws of phenomena (as some writers call them, but improperly42, since laws of causation are also laws of phenomena): that is, to uniformities, which either are not laws of causation, or in which the question of causation must for the present [Pg 433]remain undecided. The Method of Agreement is chiefly to be resorted to, as a means of suggesting applications of the Method of Difference (as in the last example the comparison of A B C, A D E, A F G, suggested that A was the antecedent on which to try the experiment whether it could produce a); or as an inferior resource, in case the Method of Difference is impracticable; which, as we before showed, generally arises from the impossibility of artificially producing the phenomena. And hence it is that the Method of Agreement, though applicable in principle to either case, is more emphatically the method of investigation on those subjects where artificial experimentation is impossible: because on those it is, generally, our only resource of a directly inductive nature; while, in the phenomena which we can produce at pleasure, the Method of Difference generally affords a more efficacious process, which will ascertain causes as well as mere20 laws.
§ 4. There are, however, many cases in which, though our power of producing the phenomenon is complete, the Method of Difference either cannot be made available at all, or not without a previous employment of the Method of Agreement. This occurs when the agency by which we can produce the phenomenon is not that of one single antecedent, but a combination of antecedents, which we have no power of separating from each other, and exhibiting apart. For instance, suppose the subject of inquiry to be the cause of the double refraction of light. We can produce this phenomenon at pleasure, by employing any one of the many substances which are known to refract light in that peculiar manner. But if, taking one of those substances, as Iceland spar for example, we wish to determine on which of the properties of Iceland spar this remarkable43 phenomenon depends, we can make no use, for that purpose, of the Method of Difference; for we cannot find another substance precisely44 resembling Iceland spar except in some one property. The only mode, therefore, of prosecuting45 this inquiry is that afforded by the Method of Agreement; by which, in fact, through a comparison of all the known substances which have the property of doubly [Pg 434]refracting light, it was ascertained that they agree in the circumstance of being crystalline substances; and though the converse46 does not hold, though all crystalline substances have not the property of double refraction, it was concluded, with reason, that there is a real connexion between these two properties; that either crystalline structure, or the cause which gives rise to that structure, is one of the conditions of double refraction.
Out of this employment of the Method of Agreement arises a peculiar modification47 of that method, which is sometimes of great avail in the investigation of nature. In cases similar to the above, in which it is not possible to obtain the precise pair of instances which our second canon requires—instances agreeing in every antecedent except A, or in every consequent except a; we may yet be able, by a double employment of the Method of Agreement, to discover in what the instances which contain A or a, differ from those which do not.
If we compare various instances in which a occurs, and find that they all have in common the circumstance A, and (as far as can be observed) no other circumstance, the Method of Agreement, so far, bears testimony48 to a connexion between A and a. In order to convert this evidence of connexion into proof of causation by the direct Method of Difference, we ought to be able, in some one of these instances, as for example A B C, to leave out A, and observe whether by doing so, a is prevented. Now supposing (what is often the case) that we are not able to try this decisive experiment; yet, provided we can by any means discover what would be its result if we could try it, the advantage will be the same. Suppose, then, that as we previously49 examined a variety of instances in which a occurred, and found them to agree in containing A, so we now observe a variety of instances in which a does not occur, and find them agree in not containing A; which establishes, by the Method of Agreement, the same connexion between the absence of A and the absence of a, which was before established between their presence. As, then, it had been shown that whenever A is present a is present, so it being now shown that when A is taken away a is removed along with it, we [Pg 435]have by the one proposition A B C, a b c, by the other B C, b c, the positive and negative instances which the Method of Difference requires.
This method may be called the Indirect Method of Difference, or the Joint50 Method of Agreement and Difference; and consists in a double employment of the Method of Agreement, each proof being independent of the other, and corroborating51 it. But it is not equivalent to a proof by the direct Method of Difference. For the requisitions of the Method of Difference are not satisfied, unless we can be quite sure either that the instances affirmative of a agree in no antecedent whatever but A, or that the instances negative of a agree in nothing but the negation52 of A. Now if it were possible, which it never is, to have this assurance, we should not need the joint method; for either of the two sets of instances separately would then be sufficient to prove causation. This indirect method, therefore, can only be regarded as a great extension and improvement of the Method of Agreement, but not as participating in the more cogent53 nature of the Method of Difference. The following may be stated as its canon:—
Third Canon.
If two or more instances in which the phenomenon occurs have only one circumstance in common, while two or more instances in which it does not occur have nothing in common save the absence of that circumstance; the circumstance in which alone the two sets of instances differ, is the effect, or the cause, or an indispensable part of the cause, of the phenomenon.
We shall presently see that the Joint Method of Agreement and Difference constitutes, in another respect not yet adverted to, an improvement upon the common Method of Agreement, namely, in being unaffected by a characteristic imperfection of that method, the nature of which still remains to be pointed54 out. But as we cannot enter into this exposition without introducing a new element of complexity55 into this long and intricate discussion, I shall postpone56 it to a subsequent chapter, and shall at once proceed to a statement of two other methods, which will complete the enumeration57 of [Pg 436]the means which mankind possess for exploring the laws of nature by specific observation and experience.
§ 5. The first of these has been aptly denominated the Method of Residues58. Its principle is very simple. Subducting61 from any given phenomenon all the portions which, by virtue62 of preceding inductions63, can be assigned to known causes, the remainder will be the effect of the antecedents which had been overlooked, or of which the effect was as yet an unknown quantity.
Suppose, as before, that we have the antecedents A B C, followed by the consequents a b c, and that by previous inductions (founded, we will suppose, on the Method of Difference) we have ascertained the causes of some of these effects, or the effects of some of these causes; and are thence apprised64 that the effect of A is a, and that the effect of B is b. Subtracting the sum of these effects from the total phenomenon, there remains c, which now, without any fresh experiments, we may know to be the effect of C. This Method of Residues is in truth a peculiar modification of the Method of Difference. If the instance A B C, a b c, could have been compared with a single instance A B, a b, we should have proved C to be the cause of c, by the common process of the Method of Difference. In the present case, however, instead of a single instance A B, we have had to study separately the causes A and B, and to infer from the effects which they produce separately, what effect they must produce in the case A B C where they act together. Of the two instances, therefore, which the Method of Difference requires,—the one positive, the other negative,—the negative one, or that in which the given phenomenon is absent, is not the direct result of observation and experiment, but has been arrived at by deduction65. As one of the forms of the Method of Difference, the Method of Residues partakes of its rigorous certainty, provided the previous inductions, those which gave the effects of A and B, were obtained by the same infallible method, and provided we are certain that C is the only antecedent to which the residual66 phenomenon c can be referred; the only agent of [Pg 437]which we had not already calculated and subducted67 the effect. But as we can never be quite certain of this, the evidence derived68 from the Method of Residues is not complete unless we can obtain C artificially and try it separately, or unless its agency, when once suggested, can be accounted for, and proved deductively from known laws.
Even with these reservations, the Method of Residues is one of the most important among our instruments of discovery. Of all the methods of investigating laws of nature, this is the most fertile in unexpected results; often informing us of sequences in which neither the cause nor the effect were sufficiently69 conspicuous70 to attract of themselves the attention of observers. The agent C may be an obscure circumstance, not likely to have been perceived unless sought for, nor likely to have been sought for until attention had been awakened71 by the insufficiency of the obvious causes to account for the whole of the effect. And c may be so disguised by its intermixture with a and b, that it would scarcely have presented itself spontaneously as a subject of separate study. Of these uses of the method, we shall presently cite some remarkable examples. The canon of the Method of Residues is as follows:—
Fourth Canon.
Subduct60 from any phenomenon such part as is known by previous inductions to be the effect of certain antecedents, and the residue59 of the phenomenon is the effect of the remaining antecedents.
§ 6. There remains a class of laws which it is impracticable to ascertain by any of the three methods which I have attempted to characterize; namely, the laws of those Permanent Causes, or indestructible natural agents, which it is impossible either to exclude or to isolate72; which we can neither hinder from being present, nor contrive14 that they shall be present alone. It would appear at first sight that we could by no means separate the effects of these agents from the effects of those other phenomena with which they cannot be prevented from coexisting. In respect, indeed, to most of the permanent causes, no such difficulty exists; since though we cannot [Pg 438]eliminate them as coexisting facts, we can eliminate them as influencing agents, by simply trying our experiment in a local situation beyond the limits of their influence. The pendulum73, for example, has its oscillations disturbed by the vicinity of a mountain: we remove the pendulum to a sufficient distance from the mountain, and the disturbance74 ceases: from these data we can determine by the Method of Difference, the amount of effect due to the mountain; and beyond a certain distance everything goes on precisely as it would do if the mountain exercised no influence whatever, which, accordingly, we, with sufficient reason, conclude to be the fact.
The difficulty, therefore, in applying the methods already treated of to determine the effects of Permanent Causes, is confined to the cases in which it is impossible for us to get out of the local limits of their influence. The pendulum can be removed from the influence of the mountain, but it cannot be removed from the influence of the earth: we cannot take away the earth from the pendulum, nor the pendulum from the earth, to ascertain whether it would continue to vibrate if the action which the earth exerts upon it were withdrawn75. On what evidence, then, do we ascribe its vibrations77 to the earth's influence? Not on any sanctioned by the Method of Difference; for one of the two instances, the negative instance, is wanting. Nor by the Method of Agreement; for though all pendulums78 agree in this, that during their oscillations the earth is always present, why may we not as well ascribe the phenomenon to the sun, which is equally a coexistent fact in all the experiments? It is evident that to establish even so simple a fact of causation as this, there was required some method over and above those which we have yet examined.
As another example, let us take the phenomenon Heat. Independently of all hypothesis as to the real nature of the agency so called, this fact is certain, that we are unable to exhaust any body of the whole of its heat. It is equally certain, that no one ever perceived heat not emanating79 from a body. Being unable, then, to separate Body and Heat, we cannot effect such a variation of circumstances as the foregoing [Pg 439]three methods require; we cannot ascertain, by those methods, what portion of the phenomena exhibited by any body is due to the heat contained in it. If we could observe a body with its heat, and the same body entirely80 divested81 of heat, the Method of Difference would show the effect due to the heat, apart from that due to the body. If we could observe heat under circumstances agreeing in nothing but heat, and therefore not characterized also by the presence of a body, we could ascertain the effects of heat, from an instance of heat with a body and an instance of heat without a body, by the Method of Agreement; or we could determine by the Method of Difference what effect was due to the body, when the remainder which was due to the heat would be given by the Method of Residues. But we can do none of these things; and without them the application of any of the three methods to the solution of this problem would be illusory. It would be idle, for instance, to attempt to ascertain the effect of heat by subtracting from the phenomena exhibited by a body, all that is due to its other properties; for as we have never been able to observe any bodies without a portion of heat in them, effects due to that heat might form a part of the very results, which we were affecting to subtract in order that the effect of heat might be shown by the residue.
If, therefore, there were no other methods of experimental investigation than these three, we should be unable to determine the effects due to heat as a cause. But we have still a resource. Though we cannot exclude an antecedent altogether, we may be able to produce, or nature may produce for us, some modification in it. By a modification is here meant, a change in it, not amounting to its total removal. If some modification in the antecedent A is always followed by a change in the consequent a, the other consequents b and c remaining the same; or vice82 versa, if every change in a is found to have been preceded by some modification in A, none being observable in any of the other antecedents; we may safely conclude that a is, wholly or in part, an effect traceable to A, or at least in some way connected with it through causation. For example, in the case of heat, though we cannot [Pg 440]expel it altogether from any body, we can modify it in quantity, we can increase or diminish it; and doing so, we find by the various methods of experimentation or observation already treated of, that such increase or diminution83 of heat is followed by expansion or contraction84 of the body. In this manner we arrive at the conclusion, otherwise unattainable by us, that one of the effects of heat is to enlarge the dimensions of bodies; or what is the same thing in other words, to widen the distances between their particles.
A change in a thing, not amounting to its total removal, that is, a change which leaves it still the same thing it was, must be a change either in its quantity, or in some of its variable relations to other things, of which variable relations the principal is its position in space. In the previous example, the modification which was produced in the antecedent was an alteration in its quantity. Let us now suppose the question to be, what influence the moon exerts on the surface of the earth. We cannot try an experiment in the absence of the moon, so as to observe what terrestrial phenomena her annihilation would put an end to; but when we find that all the variations in the position of the moon are followed by corresponding variations in the time and place of high water, the place being always either the part of the earth which is nearest to, or that which is most remote from, the moon, we have ample evidence that the moon is, wholly or partially85, the cause which determines the tides. It very commonly happens, as it does in this instance, that the variations of an effect are correspondent, or analogous, to those of its cause; as the moon moves farther towards the east, the high water point does the same: but this is not an indispensable condition; as may be seen in the same example, for along with that high water point there is at the same instant another high water point diametrically opposite to it, and which, therefore, of necessity, moves towards the west, as the moon, followed by the nearer of the tide waves, advances towards the east: and yet both these motions are equally effects of the moon's motion.
That the oscillations of the pendulum are caused by the earth, is proved by similar evidence. Those oscillations take [Pg 441]place between equidistant points on the two sides of a line, which, being perpendicular86 to the earth, varies with every variation in the earth's position, either in space or relatively87 to the object. Speaking accurately88, we only know by the method now characterized, that all terrestrial bodies tend to the earth, and not to some unknown fixed89 point lying in the same direction. In every twenty-four hours, by the earth's rotation90, the line drawn76 from the body at right angles to the earth coincides successively with all the radii91 of a circle, and in the course of six months the place of that circle varies by nearly two hundred millions of miles; yet in all these changes of the earth's position, the line in which bodies tend to fall continues to be directed towards it: which proves that terrestrial gravity is directed to the earth, and not, as was once fancied by some, to a fixed point of space.
The method by which these results were obtained, may be termed the Method of Concomitant Variations: it is regulated by the following canon:—
Fifth Canon.
Whatever phenomenon varies in any manner whenever another phenomenon varies in some particular manner, is either a cause or an effect of that phenomenon, or is connected with it through some fact of causation.
The last clause is subjoined, because it by no means follows when two phenomena accompany each other in their variations, that the one is cause and the other effect. The same thing may, and indeed must happen, supposing them to be two different effects of a common cause: and by this method alone it would never be possible to ascertain which of the suppositions is the true one. The only way to solve the doubt would be that which we have so often adverted to, viz. by endeavouring to ascertain whether we can produce the one set of variations by means of the other. In the case of heat, for example, by increasing the temperature of a body we increase its bulk, but by increasing its bulk we do not increase its temperature; on the contrary, (as in the rarefaction of air under the receiver of an air-pump,) we generally diminish it: therefore heat is not [Pg 442]an effect, but a cause, of increase of bulk. If we cannot ourselves produce the variations, we must endeavour, though it is an attempt which is seldom successful, to find them produced by nature in some case in which the pre-existing circumstances are perfectly known to us.
It is scarcely necessary to say, that in order to ascertain the uniform concomitance of variations in the effect with variations in the cause, the same precautions must be used as in any other case of the determination of an invariable sequence. We must endeavour to retain all the other antecedents unchanged, while that particular one is subjected to the requisite92 series of variations; or in other words, that we may be warranted in inferring causation from concomitance of variations, the concomitance itself must be proved by the Method of Difference.
It might at first appear that the Method of Concomitant Variations assumes a new axiom, or law of causation in general, namely, that every modification of the cause is followed by a change in the effect. And it does usually happen that when a phenomenon A causes a phenomenon a, any variation in the quantity or in the various relations of A, is uniformly followed by a variation in the quantity or relations of a. To take a familiar instance, that of gravitation. The sun causes a certain tendency to motion in the earth; here we have cause and effect; but that tendency is towards the sun, and therefore varies in direction as the sun varies in the relation of position; and moreover the tendency varies in intensity93, in a certain numerical correspondence to the sun's distance from the earth, that is, according to another relation of the sun. Thus we see that there is not only an invariable connexion between the sun and the earth's gravitation, but that two of the relations of the sun, its position with respect to the earth and its distance from the earth, are invariably connected as antecedents with the quantity and direction of the earth's gravitation. The cause of the earth's gravitating at all, is simply the sun; but the cause of its gravitating with a given intensity and in a given direction, is the existence of the sun in a given direction and at a given distance. It is not [Pg 443]strange that a modified cause, which is in truth a different cause, should produce a different effect.
Although it is for the most part true that a modification of the cause is followed by a modification of the effect, the Method of Concomitant Variations does not, however, presuppose this as an axiom. It only requires the converse proposition; that anything on whose modifications94, modifications of an effect are invariably consequent, must be the cause (or connected with the cause) of that effect; a proposition, the truth of which is evident; for if the thing itself had no influence on the effect, neither could the modifications of the thing have any influence. If the stars have no power over the fortunes of mankind, it is implied in the very terms, that the conjunctions or oppositions95 of different stars can have no such power.
Although the most striking applications of the Method of Concomitant Variations take place in the cases in which the Method of Difference, strictly so called, is impossible, its use is not confined to those cases; it may often usefully follow after the Method of Difference, to give additional precision to a solution which that has found. When by the Method of Difference it has first been ascertained that a certain object produces a certain effect, the Method of Concomitant Variations may be usefully called in, to determine according to what law the quantity or the different relations of the effect follow those of the cause.
§ 7. The case in which this method admits of the most extensive employment, is that in which the variations of the cause are variations of quantity. Of such variations we may in general affirm with safety, that they will be attended not only with variations, but with similar variations, of the effect: the proposition, that more of the cause is followed by more of the effect, being a corollary from the principle of the Composition of Causes, which, as we have seen, is the general rule of causation; cases of the opposite description, in which causes change their properties on being conjoined with one another, being, on the contrary, special and exceptional. Suppose, [Pg 444]then, that when A changes in quantity, a also changes in quantity, and in such a manner that we can trace the numerical relation which the changes of the one bear to such changes of the other as take place within our limits of observation. We may then, with certain precautions, safely conclude that the same numerical relation will hold beyond those limits. If, for instance, we find that when A is double, a is double; that when A is treble or quadruple, a is treble or quadruple; we may conclude that if A were a half or a third, a would be a half or a third, and finally, that if A were annihilated97, a would be annihilated, and that a is wholly the effect of A, or wholly the effect of the same cause with A. And so with any other numerical relation according to which A and a would vanish simultaneously98; as for instance, if a were proportional to the square of A. If, on the other hand, a is not wholly the effect of A, but yet varies when A varies, it is probably a mathematical function not of A alone, but of A and something else: its changes, for example, may be such as would occur if part of it remained constant, or varied99 on some other principle, and the remainder varied in some numerical relation to the variations of A. In that case, when A diminishes, a will be seen to approach not towards zero, but towards some other limit: and when the series of variations is such as to indicate what that limit is, if constant, or the law of its variation if variable, the limit will exactly measure how much of a is the effect of some other and independent cause, and the remainder will be the effect of A (or of the cause of A).
These conclusions, however, must not be drawn without certain precautions. In the first place, the possibility of drawing them at all, manifestly supposes that we are acquainted not only with the variations, but with the absolute quantities both of A and a. If we do not know the total quantities, we cannot, of course, determine the real numerical relation according to which those quantities vary. It is therefore an error to conclude, as some have concluded, that because increase of heat expands bodies, that is, increases the distance between their particles, therefore the distance is wholly the effect of heat, and that if we could entirely exhaust the [Pg 445]body of its heat, the particles would be in complete contact. This is no more than a guess, and of the most hazardous100 sort, not a legitimate101 induction: for since we neither know how much heat there is in any body, nor what is the real distance between any two of its particles, we cannot judge whether the contraction of the distance does or does not follow the diminution of the quantity of heat according to such a numerical relation that the two quantities would vanish simultaneously.
In contrast with this, let us consider a case in which the absolute quantities are known; the case contemplated102 in the first law of motion; viz. that all bodies in motion continue to move in a straight line with uniform velocity103 until acted upon by some new force. This assertion is in open opposition96 to first appearances; all terrestrial objects, when in motion, gradually abate104 their velocity and at last stop; which accordingly the ancients, with their inductio per enumerationem simplicem, imagined to be the law. Every moving body, however, encounters various obstacles, as friction105, the resistance of the atmosphere, &c., which we know by daily experience to be causes capable of destroying motion. It was suggested that the whole of the retardation106 might be owing to these causes. How was this inquired into? If the obstacles could have been entirely removed, the case would have been amenable107 to the Method of Difference. They could not be removed, they could only be diminished, and the case, therefore, admitted only of the Method of Concomitant Variations. This accordingly being employed, it was found that every diminution of the obstacles diminished the retardation of the motion: and inasmuch as in this case (unlike the case of heat) the total quantities both of the antecedent and of the consequent were known; it was practicable to estimate, with an approach to accuracy, both the amount of the retardation and the amount of the retarding108 causes, or resistances, and to judge how near they both were to being exhausted109; and it appeared that the effect dwindled110 as rapidly, and at each step was as far on the road towards annihilation, as the cause was. The simple oscillation of a weight suspended from a fixed point, and moved a little out of the perpendicular, which in ordinary circumstances [Pg 446]lasts but a few minutes, was prolonged in Borda's experiments to more than thirty hours, by diminishing as much as possible the friction at the point of suspension, and by making the body oscillate in a space exhausted as nearly as possible of its air. There could therefore be no hesitation111 in assigning the whole of the retardation of motion to the influence of the obstacles; and since, after subducting this retardation from the total phenomenon, the remainder was an uniform velocity, the result was the proposition known as the first law of motion.
There is also another characteristic uncertainty affecting the inference that the law of variation which the quantities observe within our limits of observation, will hold beyond those limits. There is of course, in the first instance, the possibility that beyond the limits, and in circumstances therefore of which we have no direct experience, some counteracting112 cause might develop itself; either a new agent, or a new property of the agents concerned, which lies dormant113 in the circumstances we are able to observe. This is an element of uncertainty which enters largely into all our predictions of effects; but it is not peculiarly applicable to the Method of Concomitant Variations. The uncertainty, however, of which I am about to speak, is characteristic of that method; especially in the cases in which the extreme limits of our observation are very narrow, in comparison with the possible variations in the quantities of the phenomena. Any one who has the slightest acquaintance with mathematics, is aware that very different laws of variation may produce numerical results which differ but slightly from one another within narrow limits; and it is often only when the absolute amounts of variation are considerable, that the difference between the results given by one law and by another becomes appreciable114. When, therefore, such variations in the quantity of the antecedents as we have the means of observing, are small in comparison with the total quantities, there is much danger lest we should mistake the numerical law, and be led to miscalculate the variations which would take place beyond the limits; a miscalculation which would vitiate any conclusion respecting [Pg 447]the dependence115 of the effect upon the cause, that could be founded on those variations. Examples are not wanting of such mistakes. "The formul?," says Sir John Herschel,[33] "which have been empirically deduced for the elasticity116 of steam, (till very recently,) and those for the resistance of fluids, and other similar subjects," when relied on beyond the limits of the observations from which they were deduced, "have almost invariably failed to support the theoretical structures which have been erected117 on them."
In this uncertainty, the conclusion we may draw from the concomitant variations of a and A, to the existence of an invariable and exclusive connexion between them, or to the permanency of the same numerical relation between their variations when the quantities are much greater or smaller than those which we have had the means of observing, cannot be considered to rest on a complete induction. All that in such a case can be regarded as proved on the subject of causation is, that there is some connexion between the two phenomena; that A, or something which can influence A, must be one of the causes which collectively determine a. We may, however, feel assured that the relation which we have observed to exist between the variations of A and a, will hold true in all cases which fall between the same extreme limits; that is, wherever the utmost increase or diminution in which the result has been found by observation to coincide with the law, is not exceeded.
The four methods which it has now been attempted to describe, are the only possible modes of experimental inquiry—of direct induction à posteriori, as distinguished118 from deduction: at least, I know not, nor am able to imagine, any others. And even of these, the Method of Residues, as we have seen, is not independent of deduction; though, as it also requires specific experience, it may, without impropriety, be included among methods of direct observation and experiment.
These, then, with such assistance as can be obtained from [Pg 448]Deduction, compose the available resources of the human mind for ascertaining119 the laws of the succession of phenomena. Before proceeding120 to point out certain circumstances, by which the employment of these methods is subjected to an immense increase of complication and of difficulty, it is expedient121 to illustrate122 the use of the methods, by suitable examples drawn from actual physical investigations123. These, accordingly, will form the subject of the succeeding chapter.
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1
illustrating
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| 给…加插图( illustrate的现在分词 )； 说明； 表明； （用示例、图画等）说明 | |
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inquiries
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| n.调查( inquiry的名词复数 )；疑问；探究；打听 | |
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phenomena
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| n.现象 | |
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investigation
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| n.调查，调查研究 | |
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inquiry
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| n.打听，询问，调查，查问 | |
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ascertain
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| vt.发现，确定，查明，弄清 | |
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greasy
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| adj. 多脂的，油脂的 | |
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random
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| adj.随机的；任意的；n.偶然的(或随便的)行动 | |
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deposition
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| n.免职，罢官；作证；沉淀；沉淀物 | |
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fusion
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| n.溶化；熔解；熔化状态，熔和；熔接 | |
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solidification
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| 凝固 | |
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induction
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| n.感应，感应现象 | |
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quartz
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| n.石英 | |
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contrive
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| vt.谋划，策划；设法做到；设计，想出 | |
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remains
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| n.剩余物，残留物；遗体，遗迹 | |
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unconditional
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| adj.无条件的，无限制的，绝对的 | |
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uncertainty
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| n.易变，靠不住，不确知，不确定的事物 | |
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immediate
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| adj.立即的；直接的，最接近的；紧靠的 | |
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ascertained
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| v.弄清，确定，查明( ascertain的过去式和过去分词 ) | |
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mere
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| adj.纯粹的；仅仅，只不过 | |
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adverted
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| 引起注意（advert的过去式与过去分词形式） | |
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potent
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| adj.强有力的，有权势的；有效力的 | |
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procure
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| vt.获得，取得，促成；vi.拉皮条 | |
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noted
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| adj.著名的，知名的 | |
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elimination
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| n.排除，消除，消灭 | |
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analogous
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| adj.相似的；类似的 | |
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exclusion
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| n.拒绝，排除，排斥，远足，远途旅行 | |
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experimentation
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| n.实验，试验，实验法 | |
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peculiar
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| adj.古怪的，异常的；特殊的，特有的 | |
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strictly
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| adv.严厉地，严格地；严密地 | |
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inaccessibly
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| Inaccessibly | |
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perfectly
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| adv.完美地，无可非议地，彻底地 | |
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alteration
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| n.变更，改变；蚀变 | |
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plunged
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| v.颠簸( plunge的过去式和过去分词 )；暴跌；骤降；突降 | |
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fully
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| adv.完全地，全部地，彻底地；充分地 | |
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suffocation
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| n.窒息 | |
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interim
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| adj.暂时的，临时的；n.间歇，过渡期间 | |
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immersion
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| n.沉浸；专心 | |
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conclusively
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| adv.令人信服地，确凿地 | |
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attained
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| （通常经过努力）实现( attain的过去式和过去分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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affected
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| adj.不自然的，假装的 | |
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improperly
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| 不正确地，不适当地 | |
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remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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precisely
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| adv.恰好，正好，精确地，细致地 | |
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prosecuting
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| 检举、告发某人( prosecute的现在分词 )； 对某人提起公诉； 继续从事（某事物）； 担任控方律师 | |
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converse
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| vi.谈话，谈天，闲聊；adv.相反的，相反 | |
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modification
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| n.修改，改进，缓和，减轻 | |
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testimony
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| n.证词；见证，证明 | |
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previously
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| adv.以前，先前（地） | |
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joint
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| adj.联合的，共同的；n.关节，接合处；v.连接，贴合 | |
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corroborating
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| v.证实，支持（某种说法、信仰、理论等）( corroborate的现在分词 ) | |
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negation
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| n.否定；否认 | |
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cogent
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| adj.强有力的，有说服力的 | |
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pointed
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| adj.尖的，直截了当的 | |
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complexity
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| n.复杂(性)，复杂的事物 | |
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postpone
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| v.延期，推迟 | |
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enumeration
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| n.计数，列举；细目；详表；点查 | |
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residues
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| n.剩余，余渣( residue的名词复数 )；剩余财产；剩数 | |
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residue
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| n.残余，剩余，残渣 | |
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subduct
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| v.减去，除去 | |
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subducting
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| v.下拉，（使）下降； 除去，撤去 | |
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virtue
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| n.德行，美德；贞操；优点；功效，效力 | |
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inductions
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| 归纳（法）( induction的名词复数 )； （电或磁的）感应； 就职； 吸入 | |
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apprised
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| v.告知，通知( apprise的过去式和过去分词 )；评价 | |
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deduction
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| n.减除，扣除，减除额；推论，推理，演绎 | |
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residual
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| adj.复播复映追加时间；存留下来的，剩余的 | |
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subducted
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| v.下拉，（使）下降； 除去，撤去 | |
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derived
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| vi.起源；由来；衍生；导出v.得到( derive的过去式和过去分词 )；（从…中）得到获得；源于；（从…中）提取 | |
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sufficiently
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| adv.足够地，充分地 | |
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conspicuous
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| adj.明眼的，惹人注目的；炫耀的，摆阔气的 | |
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awakened
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| v.（使）醒( awaken的过去式和过去分词 )；（使）觉醒；弄醒；（使）意识到 | |
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isolate
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| vt.使孤立，隔离 | |
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pendulum
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| n.摆，钟摆 | |
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disturbance
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| n.动乱，骚动；打扰，干扰；(身心)失调 | |
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withdrawn
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| vt.收回；使退出；vi.撤退，退出 | |
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drawn
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| v.拖，拉，拔出；adj.憔悴的，紧张的 | |
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vibrations
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| n.摆动( vibration的名词复数 )；震动；感受；（偏离平衡位置的）一次性往复振动 | |
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pendulums
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| n.摆，钟摆( pendulum的名词复数 )；摇摆不定的事态（或局面） | |
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emanating
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| v.从…处传出，传出( emanate的现在分词 )；产生，表现，显示 | |
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entirely
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| ad.全部地，完整地；完全地，彻底地 | |
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divested
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| v.剥夺( divest的过去式和过去分词 )；脱去（衣服）；2。从…取去…；1。（给某人）脱衣服 | |
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vice
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| n.坏事；恶习；[pl.]台钳，老虎钳；adj.副的 | |
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diminution
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| n.减少；变小 | |
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contraction
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| n.缩略词，缩写式，害病 | |
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85
partially
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| adv.部分地，从某些方面讲 | |
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perpendicular
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| adj.垂直的，直立的；n.垂直线，垂直的位置 | |
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relatively
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| adv.比较...地，相对地 | |
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accurately
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| adv.准确地，精确地 | |
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fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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rotation
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| n.旋转；循环，轮流 | |
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radii
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| n.半径；半径（距离）( radius的名词复数 )；用半径度量的圆形面积；半径范围；桡骨 | |
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requisite
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| adj.需要的，必不可少的；n.必需品 | |
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intensity
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| n.强烈，剧烈；强度；烈度 | |
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modifications
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| n.缓和( modification的名词复数 )；限制；更改；改变 | |
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oppositions
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| （强烈的）反对( opposition的名词复数 )； 反对党； （事业、竞赛、游戏等的）对手； 对比 | |
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opposition
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| n.反对，敌对 | |
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annihilated
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| v.（彻底）消灭( annihilate的过去式和过去分词 )；使无效；废止；彻底击溃 | |
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simultaneously
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| adv.同时发生地，同时进行地 | |
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varied
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| adj.多样的，多变化的 | |
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hazardous
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| adj.(有)危险的，冒险的；碰运气的 | |
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legitimate
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| adj.合法的，合理的，合乎逻辑的；v.使合法 | |
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contemplated
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| adj. 预期的 动词contemplate的过去分词形式 | |
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velocity
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| n.速度，速率 | |
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abate
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| vi.(风势，疼痛等)减弱，减轻，减退 | |
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friction
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| n.摩擦，摩擦力 | |
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retardation
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| n.智力迟钝，精神发育迟缓 | |
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amenable
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| adj.经得起检验的；顺从的；对负有义务的 | |
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retarding
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| 使减速( retard的现在分词 )； 妨碍； 阻止； 推迟 | |
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exhausted
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| adj.极其疲惫的，精疲力尽的 | |
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dwindled
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| v.逐渐变少或变小( dwindle的过去式和过去分词 ) | |
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hesitation
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| n.犹豫，踌躇 | |
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counteracting
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| 对抗，抵消( counteract的现在分词 ) | |
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113
dormant
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| adj.暂停活动的；休眠的；潜伏的 | |
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appreciable
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| adj.明显的，可见的，可估量的，可觉察的 | |
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115
dependence
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| n.依靠，依赖；信任，信赖；隶属 | |
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elasticity
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| n.弹性，伸缩力 | |
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117
ERECTED
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| adj. 直立的，竖立的，笔直的 vt. 使 ... 直立，建立 | |
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118
distinguished
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| adj.卓越的，杰出的，著名的 | |
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ascertaining
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| v.弄清，确定，查明( ascertain的现在分词 ) | |
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proceeding
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| n.行动，进行，(pl.)会议录，学报 | |
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expedient
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| adj.有用的，有利的；n.紧急的办法，权宜之计 | |
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illustrate
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| v.举例说明，阐明；图解，加插图 | |
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investigations
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| （正式的）调查( investigation的名词复数 )； 侦查； 科学研究； 学术研究 | |
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