164 MEMOIR OF AUGUSTUS DE MORGAN. 



1847. of limits in the Differential Calculus. But his views of 

 Berkeley's the psychology of Berkeley as compared with his own are 



philosophy. r J 5r j j_- 



more generally interesting. 



In the chapter On Objects, Ideas, and Names in the 

 Formal Logic, the writer says : 



That our minds, souls, or thinking powers, use what name 

 we may, exist, is the thing of which of all others we are most 

 certain, each for himself. Next to this nothing can be more 

 certain to us each for himself than that other things also exist ; 

 other minds, our own bodies, the whole world of matter. But 

 between the character of these two certainties there is a vast 

 difference. Any one who should deny his own existence would, 

 if serious, be held beneath argument ; he does not know the 

 meaning of his words, or he is false or mad. But if the same 

 man should deny that anything exists except himself that is, if 

 he should affirm the whole creation to be a dream of his own 

 mind he would be absolutely unanswerable. If I (who know 

 he is wrong, for I am certain of my own existence) argue with 

 him, and reduce him to silence, it is no more than might happen 

 in his dream. . . . 



A celebrated metaphysician, Berkeley, maintained that with 

 regard to matter the above is the state of the case ; that our 

 impressions of matter are only impressions communicated by the 

 Creator without any intervening cause of communication. 



Our most convincing communicable proof of the existence of 

 other things is, not the appearance of objects, but the necessity 

 of admitting that there are other minds beside our own. The 

 external inanimate objects might be creations of our own 

 thought, or thinking and perceptive functions. They are so 

 sometimes in the case of insanity, in which the mind has fre- 

 quently the appearance of making the whole or part of its own 

 external world. But when we see other beings performing 

 similar functions to those which we ourselves perform, we come so 

 irresistibly to the conclusion that there must be other sentients 

 like ourselves, that we should rather compare a person who 

 doubted it to one who denied his own existence, than to one 

 who really denied the external existence of the material world. 



In his interleaved copy of Formal Logic is a pencil 

 note alongside of the foregoing : 



To read Berkeley so as to give him a fair chance, some one 



