6f)0 PSYCHO LOO Y. 



things which we consider to be of this kind or of that, but 

 later learn that they have none of the kind's properties, that 

 they do not belong to the kind's kind. Instead, however, of 

 correcting the principle by these cases, we correct the cases 

 by the principle. We say that if the thing we named an M 

 has not M's properties, then we were either mistaken in call- 

 ing it an M, or mistaken about M's properties ; or else that it 

 is no longer M, but has changed. But we never say that it 

 is an M without M's properties ; for by conceiving a thing as 

 of the kind M I mean that it shall have M's properties, be of 

 M's kind, even ihough I should never be able to find in the 

 real world anything which is an M. The principle emanates 

 from my perception of what a lot of successive is's mean. 

 This perception can no more be confirmed by one set, or 

 weakened by another set, of outer facts, than the perception, 

 that black is not white can be confirmed by the fact that 

 snow never blackens, or weakened by the fact that photog- 

 rapher's paper blackens as soon as you lay it in the sun. 



The abstract scheme of successive predications, extended 

 indefinitely, with all the possibilities of substitution Avhich 

 it involves, is thus an immutable system of truth which 

 flows from the very structure and form of our thinking. 

 If any real terms ever do fit into such a scheme, they 

 will obey its laws ; ivhetlier they do is a question as to 

 nature's facts, the answer to which can only be empiri- 

 cally ascertained. Formal logic is the name of the Science 

 which traces in skeleton form all the remote relations 

 of terms connected by successive ^'s's with each other, 

 and enumerates their possibilities of mutual substitution. 

 To our principle of mediate subsumption she has given 

 various formulations, of which the best is perhaps this 

 broad expression, that the same can he substituted for the same 

 in any mental operation* 



The ordinary logical series contains but three terms 



* Realities fall under this only so far as they prove to be the same. So 

 far as they cannot be substituted for each other, for the purpose in hand, 

 so far they are not the same ; though for other purposes and in other 

 respects they might be substituted, and then be treated as the same. Apart 

 from purpose, of course, :io realities ever are absolutely and exactly the 

 same. 



