142 INDUCTION. 



To ascertain whether, and in what, two phenomena re- 

 semble or differ, 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 employ the indirect 

 means of reasoning and general propositions. When we can- 

 not 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 formula, "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 

 dissimilarities, and is the sum total of what Logic has to teach 

 on the subject. 



An undue extension of this remark induced Locke to con- 

 sider reasoning itself as nothing but the comparison of two 

 ideas through the medium of a third, and knowledge as the 

 perception of the agreement or disagreement of two ideas : 

 doctrines which the Condillac school blindly adopted, without 

 the qualifications and distinctions with which they were 

 studiously guarded by their illustrious author. Where, indeed, 

 the agreement or disagreement (otherwise called resemblance 

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

 determined, as is the case particularly in the sciences of quan- 

 tity 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 per- 

 ception of agreement or disagreement, but of a series of 

 physical occurrences, a succession of sensations. Locke's 

 definitions of knowledge and of reasoning required to be 

 limited to our knowledge of, and reasoning about, resem- 

 blances. Nor, even when thus restricted, are the propositions 

 strictly correct ; since the comparison is not made, as he repre- 

 sents, between the ideas of the two phenomena, but between 



