INFERENCE OR DEDUCTION.

519

clothing, the prevention of draughts, or the admission of cool air, are the trains of thought usually suggested by the various facts of congelation, liquefaction, &c.; to enter upon the other trains is the result of a special training and endowment, the explanation of which according to general laws of mind has been one of the aims of our protracted examination of the human intellect.

36. Inference, Deduction, Ratiocination, Syllogism, Application or Extension of Inductions.—I have repeatedly urged the value of the identifying process in extending our knowledge, by transferring all that has been ascertained in some one case to every other case of the same description. This operation is described under all the above titles. It is an Inference, a Deduction, a step of Reasoning, the Extension of an Affirmation from the known to the unknown. The discovery of a true identity* between the new cases and the old is a justification of this transference of properties. Having, for example, observed in innumerable cases that human beings go through a course of birth, maturity, decay, and death, we transfer their fate to those now alive, and we declare beforehand that each and all of these will go through the same course; this is to make an inference, to reason, to apply our knowledge to new cases, to know the future from the past, the absent from the present. So, when we land on the banks of a strange river, we instantly proceed on the assumption that this river has its origin in high lands, its destination in the sea, and has at its mouth a deposit of mud of larger or smaller dimensions. The little that we see of the river, by walking a few miles along its bank is enough to identify it with the rivers already known to us, or with our general notion, or abstract idea, or definition of a river, and on this identity we forthwith transfer all our experience connected with rivers in general, and all their conjoined phenomena, to the newly

It is not within the scope of this treatise to explain fully the nature of the evidence which the scientific man requires in order to be satisfied that a supposed identity is real, true, or genuine, or a sufficient basis for deductive inference. Such an explanation is most amply supplied in the work mentioned in the next paragraph.

occurring individual case. When our knowledge comes thus to transcend our actual experience, an inference or act of Deduction or ratiocination is performed.

Mr. John Stuart Mill, in his System of Logic, has shown, I think, conclusively, that the basis of all inference is a transition from particulars to particulars, and not, as usually supposed, the application of a general affirmation to the special affirmations included in it. In fact he maintains that when we say all men are mortal, we have already inferred the utmost that it is possible to infer; for out of our experience of the men that have lived and died, we have constructed an assertion applying to all men now living and all that are yet to be born, so that no further deduction remains to be made; the applying of this affirmation to a particular individual or tribe, as to the present inhabitants of London, or the present Emperor of China, is not an inference, it is but to enunciate in detail what has been already enunciated in the gross or total—it is not to make any new step, or to take up with any new piece of information, any new belief. Hence to syllogise is only to go through a form of reasoning; it is to take precaution against one particular source of mistake, namely, the mistake of wrongfully including an individual in a general class. If we say all men are mortal, therefore the angel Gabriel will die, the badness of the reasoning will be exposed by giving it the syllogistic form, thus: all men are mortal, the angel Gabriel is a man, Gabriel will die. Here, by completing the form, we see what assertions we make previously to drawing the conclusion, and that while the major or principal proposition, all men are mortal, is true, the second, or minor proposition, that Gabriel belongs to the class of men, must also be verified. The stress of the syllogism lies not in extending our knowledge to new cases, for this extension is complete when a general proposition is risked, but in making sure of the relevancy, the applicability to the case in hand; it requires a solemn, deliberate, overt assertion of the identity of the specific case with the cases contemplated in the already generalized affirmation; and we presume that a person in making this assertion feels that he ought to be quite certain of

EXTENSION OF THE KNOWN TO THE UNKNOWN. 521

its accuracy before committing himself to such a broad avowal as this formality brings him to.

This process of inference or extension of properties, therefore, evidently comes of the identifying faculty, by which the new cases and the old are brought together in the view. If the question be, given a certain number of particulars, where a natural law is repeated, to discover other particulars whereto we may extend or apply the law and so reveal new characters in those particulars, these new cases must be summoned to the view by a stroke of similarity. For example, Newton observed in various cases that when a transparent body is largely made up of combustible matter, as an oil or a resin, that it bends light to an unusual degree; in other words, he made an induction of particulars where combustibility of substance and excessive bending of light were conjoined properties. He next bethought himself of any other substances, besides those in the immediate view that possessed one of these properties, and his recollection of the refracting power of the diamond responded to his call by a stroke of similarity; he thereupon extended to the diamond the other property, namely, combustibility of material, or inferred what no one had ever experienced, that the diamond is a combustible substance, a singular exception to the class of precious stones. This active obtrusion of observed coincidences upon all parallel cases, this laying out the mind for the suggestion of new particulars to have the observed properties thrust upon them, is one of the ways of extending the domain of knowledge. The enquirer has got in his hand a clue, and makes a business of following it out wherever he can find an opening; he has made his induction, and lies in wait for occasions of pushing it out into deductions. In this endeavour his identifying faculty will avail him much; it will make him as it were keen-scented for everything in the memory of the past that bears a shadow of resemblance to his case; the recollections that in an obtuse mind would lie unawakened by the magnetism of similarity, in the mind of the other start out one by one for examination and choice; and in this lies the harvest home of the man of intellect.

We can next suppose the opposite case; given a dark spot, an obscure phenomenon, to illuminate it by bringing forward parallels or identities among phenomena that are clear and intelligible, supposing such to have actually occurred at some time or other, but in a connexion altogether remote from the present difficulty, so that only the force of similarity can. bring them up. The position of the enquirer is now altered; nevertheless the intellectual operation is the same; to summon the clear to illuminate the dark, or to summon the dark to be illuminated by the clear, must alike proceed on a felt identity, which identity is both the mental link of attraction and the circumstance that justifies the transference of information from the one to the other. Of the instances already brought forward several would correspond to this supposition; but instead of recurring to these we will cite the great identity struck out by Franklin, between the thunder and lightning of the sky, and electricity as shown on the common electrical machine. Next to the discovery of gravitation, this is perhaps the most remarkable fetch of remote identification that the history of science presents. The phenomenon of the thundery discharge was an exceedingly obscure and mysterious action; the natural obscurity of the case was farther increased by the emotions that it habitually inspired in men's minds, for nothing is more difficult than to identify, on a mere intellectual similarity, what excites deep emotions (especially fear) with what excites no emotion at all. Only a cool intellectual nature such as distinguished Franklin was a match for a case like this. He could face the evolution of a thunder-storm, and watch it with all the calmness that he would have shown in an ordinary philosophical experiment, deliberately bethinking himself the while of any parallel phenomenon wherewith he could identify and illustrate it. Had he taken up the inquiry a century earlier his attempt would have been in vain; for among all the scientific facts that could have crossed his view in the middle of the seventeenth century, no degree of identifying energy would have been able to summon up a single one to compare with the case in hand. In the eighteenth century his position was

REASONING BY ANALOGY.

523

different; the electrical machine was a familiar instrument, and an intelligible account of its phenomena had been rendered; and these phenomena had been studied by Franklin, and were vividly impressed on his mind. To his cool eye gazing on the storm, the forked lightning identified itself (in the midst of a diversity that few other minds could have broken through) with the spark of an electrical discharge. This was indeed the only feature of resemblance, unless a favourable accident had revealed some other coincidence, such as the existence of an electrical charge in the clouds before a storm; and I consider the identification to have been a stroke of similarity of the very first order. It took all the preparation of an accurate study of the parallel subject of common electricity, with the passionless temperament and the strong intellect of Franklin, to achieve such a conquest over the obscurity that shrouds the atmospheric agencies. The identity once struck was duly verified, and proved to be a real and not a superficial or apparent sameness; being, in fact, the same natural power showing itself in widely different situations. Then came all the deductive applications; the circumstances known to accompany and precede the discharge of a Leyden jar could be transferred to the electrical storm;-the charging of the clouds with one electricity and the earth with an opposite, the increase of electrical tension to the pitch that an intervening insulator could no longer restrain, the shock of discharge, were seen through the medium of the familiar parallel to be the history of the lightning and thunder of the sky. Every new fact ascertained in the machine could thenceforth be extended to the atmosphere; what could not be discovered there at first hand could still be known through the medium of deductive inference.

The subject of electricity could furnish me with many other examples of scientific identification on a great scale, but my limits forbid me to dwell upon them.

37. Reasoning by Analogy. The three foregoing sections comprehend the leading processes of scientific discovery; every great step in science is either an Abstraction, an Induction, or a Deduction. But resort is occasionally had to Analogy, as a