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as a contribution to the science of logic, that is, to the analysis of the mental processes concerned in reasoning, the new doctrine appears to me, I confess, not merely superfluous, but erroneous; since the form in which it clothes the propositions does not, like the ordinary form, express what is in the mind of the speaker when he enunciates the proposition. I cannot think Sir William Hamilton right in maintaining that the quantity of the predicate is always understood in thought.' It is implied, but is not present in the mind of the person who asserts the proposition." Again, he says of Mr. De Morgan's ingenious logical discoveries, to which every logical writer is so deeply indebted:-" Since it is undeniable that inferences, in the cases examined by Mr. De Morgan, can legitimately be drawn, and that the ordinary theory takes no account of them, I will not say that it was not worth while to show in detail how these also could be reduced to formulæ as rigorous as those of Aristotle. What Mr. De Morgan has done was worth doing once (perhaps more than once), as a school exercise; but I question if its results are worth studying and mastering for any practical purpose." In these and many other places Mr. Mill shows a lamentable want of power of appreciating the principles involved in the
quantification of the predicate. As regards the most original discoveries of Dr. Boole, there is not, so far as I have been able to discover, a single word in Mr. Mill's edition of his "Logic" published in 1862, to indicate that he was conscious of the publication of Mr. Boole's "Mathematical Analysis." in 1849, and of his great work, "The Laws of Thought," in 1854. Although accounted a disciple and potent supporter of the doctrines of Jeremy Bentham, he appears unaware that the doctrine of the quantification of the predicate is traceable to his great master, or at all events to the work of a nephew founded upon the manuscripts of Bentham.
60. I ought not to omit to notice that Dr. Thomson substantially adopts the principle of substitution in treating of what he calls the syllogism of analogy. He states the canon in the following manner :-"The same attributes may be assigned to distinct but similar things, provided they can be shown to accompany the points of resemblance in the things and not the points of difference." This means that one thing may be substituted for another like to it, provided that their likeness really extends to the point in question, which can often only be ascertained with more or less probability 1 "Laws of Thought," fifth ed. p. 251.
by inductive inquiry. He adds, that the expression of the agreement must consist of a qualified judgment of identity, or a proposition of the form U, by which symbol he indicates a proposition denoted in this tract by the expression A= B. This exactly agrees with my view of the matter.
61. The principle of substitution of similars seems to throw a clear light upon the infinite importance of classification. For classification consists in arranging things, either in the mind or in cabinets of specimens, according to their resemblances, and the best classification is that which exhibits the most numerous and extensive resemblances. The purpose and effect of such arrangement evidently is, that we may apply to all members of a class whatever we know of any member, so far as it is a member. All the members of a class are mutual substitutes for each other as regards their common characteristics, and a natural classification is that which gives the greatest probability that characters as yet unexamined will exhibit agreements corresponding to those which are examined. Classification is thus the infinitely useful mode of multiplying knowledge, by rendering knowledge of particulars as general as possible, or of indicating the greatest possible number of substitutions which may give rise to acts of inference.
62. I need hardly point out that not only in our reasonings, but in our acts in common life, we observe the principle of similarity. Any new kind of action or work is performed with doubt and difficulty, because we have no knowledge derived from a similar case to guide us. But no sooner has the work been performed once or twice with success than much of the difficulty vanishes, because we have acquired all the knowledge which will guide us in similar cases. Mankind, too, have an instinctive respect for precedents, feeling that, however we act in one particular case, we ought to act / similarly in all similar cases, until strong reason or necessity obliges us to make a new precedent. The whole practice of law in English courts, if not in all others, consists in deciding all new causes according to the rule established in the most nearly similar former causes, provided any can be found sufficiently similar. No ruler, too, but an absolute tyrant can perform any public act but under the responsibility of being called upon to perform a similar act, or make a similar concession, in similar circumstances.
63. At the present day, for instance, the Government is called upon to take charge of the telegraphs and railways, because great benefit has resulted from their management of the post-office. It is
implied in this demand that the telegraphs and railways resemble or are even identical with the post-office, in those points which render Government control beneficial, and the public mind inevitably leaps from one thing to anything which appears similar. The whole question turns, of course, upon the degree and particular nature of the similarity. Granting that there is sufficient analogy between the telegraph and the post-office to render the Government purchase of the former desirable, we must not favour so gigantic an enterprise as the purchase of the railways until it is clearly made out that their successful management depends upon principles of economy exactly similar to the case of the post-office.
64. The great immediate question of the day is the Disestablishment of the Irish Church. The opponents of the measure argue against it by the indirect argument, that if the Irish Church ought to be disestablished, so ought the English Church; but as this ought not, neither ought the Irish Church. They are answered by pointing out that the Irish and English Churches are not similarly situated; the one possesses the sympathy of the great body of the people, and the other does not. This is an all-important point, which prevents our applying to one what we apply to the other. But