362 INDUCTION. 



this has been done, things in reality very dissimilar often appear undis- 

 guishably alike. Two lines of very unequal length will appear about 

 equal when lying in different directions ; but place them parallel, with 

 their further extremities even, and if you look at the nearer extremities, 

 their inequality becomes a matter of direct perception. 



To ascertain whether, and in what, two phenomena resemble or dif- 

 fer, is not always, therefore, so easy a thing as it might at first appear. 

 When the two cannot be brought into juxtaposition, or not so that the 

 observer is able to compare their several parts in detail, he must em- 

 ploy the indirect means of reasoning and general propositions. When 

 we cannot bring two straight lines together, to determine whether they 

 are equal, we do it by the physical aid of a foot rule applied first to 

 one and then to the other, and the logical aid of the general proposition 

 or foiTuula, "Things which are equal to the same thing are equal to 

 one another." The comparison of two things through the intervention 

 of a third thing, when their direct comparison is impossible, is the ap- 

 propriate scientific process for ascertaining resemblances and dissimi- 

 larities, and is the sum total of what Logic has to teach on the subject. 



An undue extension of these views induced Locke to consider 

 reasoning itself as nothing but the comparison of two ideas through 

 the medium of a third, and knowledge as the perception of the agree- 

 ment or disagi-eement of two ideas : doctrines which the Condillac 

 school blindy adopted, without the qualifications and distinctions with 

 which they were studiously guarded by their illustrious author. 

 Where, indeed, the agreement or disagi-eement (otherwise called re- 

 semblance or dissimilarity) of any two things is the very matter to be 

 determined, as is the case particularly in the sciences of quantity and 

 extension, there the process by which a solution, if not attainable by 

 direct perception, must be indirectly sought, consists in comparing 

 these two things through the medium of a third. But this is far from 

 being true of all inquiries. The knowledge that bodies fall to the 

 ground is not a perception of agreement or disagreement, but of a 

 series of physical occurrences, a succession of sensations. Locke's defi- 

 nitions of knowledge and. of reasoning required to be limited to oui- 

 knowledge of, and reasoning about, Resemblances. Nor, even when 

 thus restricted, are the propositions strictly connect ; since the com- 

 parison is not made, as he represents, between the ideas of the two 

 phenomena, but between the phenomena themselves. This mistake 

 has been pointed out in an earlier patt of our inquiry,* and we traced 

 it to an imperfect conception of what takes place in mathematics, 

 where very often the comparison is really made between the ideas, 

 without any appeal to the outward senses ; only, however, because in 

 mathematics a comparison of the ideas is strictly equivalent to a com- 

 parison of the phenomena themselves. Wliere, as in the case of num- 

 bers, lines, and figures, our idea of an object is a complete picture of 

 the object, so far as respects the matter in hand ; we can of course 

 learn from the picture, whatever could be learnt fi-om the object itself 

 by mere contemplation of it as it exists at the particular instant when 

 the picture is taken. No mere contemplation of gunpowder would 

 ever teach us that a spark would make it explode, nor, consequently, 

 would the contemplation of the idea of gunpowder do so : but the mere 



* Supra, pp. 59, 154. 



