192 Instinct. 



be sceptical, or to Insist upon absolute proof of their 

 agreement or likeness, in any given case. 



The proof of identity of sensations and sense- 

 perceptions, can be only inferential, strengthened, 

 indeed, by our belief in the uniformity, which we 

 see running throughout our physical structures, so 

 far as the examination can be made, by the aid of 

 the senses. 



Because animals cannot speak, it seems, at first 

 thought, a more difficult thing to compare man 

 with an animal, than it is to compare man with 

 man. And so it is, in some respects ; as the men- 

 tal operations can best be revealed through lan- 

 guage, and some of them, only in this way. But 

 the difference, in the two cases, is by no means so 

 great as it, at first, appears. We must in both cases, 

 infer the correspondence of sensations and men- 

 tal states, by certain effects. Man can aid us by 

 language ; but, on the other hand, an animal has 

 no metaphysical theory that comes in to disturb 

 his sensations, or his acts, as consequent upon those 

 sensations. So it may fairly be assumed that the 

 honesty of an animal, in acting free from all theo- 

 ries, and all knowledge that he is under examina- 

 tion, may be a fair offset against the gift of speech, 

 as an aid in investigating the sensations and con- 

 ceptions of our fellow-men. 



As our object now is, to make a fair comparison 

 of man with the highest and best known, of the 

 lower animals, we begin with their bodies. 



The structure of all vertebrates is essentially 

 the same, and varied, only in accordance with the 



