DEMONSTRATION, AND NECESSARY TRUTHS. 183 



person who has not thought steadily about Substance, it may not appear 

 inconceivable, that by chemical operations, we should generate new matter^ 

 or destroy matter which already exists."* Necessary truths, therefore, are 

 not those of which we can not conceive, but " those of which we can not 

 distinctly conceive, the contrary."f So long as our ideas are indistinct al- 

 together, we do not know what is or is not capable of being distinctly 

 conceived ; but, by the ever increasing distinctness- with which scientific 

 men apprehend the general conceptions of science, they in time come to 

 perceive that there are certain laws of nature, which, though historically 

 and as a matter of fact they were learned from experience, we can noty» 

 now that we know them, distinctly conceive to be other than they are. 



The account which I should give of this progress of the scientific min 

 is somewhat different. After a general law of nature has been ascertained, 

 men's minds do not at first acquire a complete facility of familiarly repre- 

 senting to themselves the phenomena of nature in the character which that 

 law assigns to them. The habit which constitutes the scientific cast of 

 mind, that of conceiving facts of all descriptions conformably to the laws 

 which regulate them — phenomena of all descriptions according to the re- 

 lations which have been ascertained really to exist between them; this hab- 

 it, in the case of newly-discovered relations, comes only by degrees. So 

 long as it is not thoroughly formed, no necessary character is ascribed to 

 the new truth. But in time, the philosopher attains a state of mind in 

 which his mental picture of nature spontaneously represents to him all the 

 phenomena with which the new theory is concerned, in the exact light in 

 which the theory regards them : all images or conceptions derived from' 

 any other theory, or from the confused view of the facts which is anterior 

 to any theory, having entirely disappeared from his mind. The mode of 

 representing facts which results from the theory, has now become, to his 

 faculties, the only natural mode of conceiving them. It is a known truth, 

 that a prolonged habit of arranging phenomena in certain groups, and ex- 

 plaining them by means of certain principles, makes any otlier arrangement 

 or explanation of these facts be felt as unnatural : and it may at last be- 

 come as difficult to him to represent the facts to himself in any other mode, 

 as it often was, originally, to represent them in that mode. 



But, further (if the theory is true, as we are supposing it to be), any 

 other mode in which he tries, or in which he was formerly accustomed, 

 to represent the phenomena, will be seen by him to be inconsistent with 

 the facts that suggested the new theory — facts which now form a part of 

 his mental picture of nature. And since a contradiction is always incon- 

 ceivable, his imagination rejects these false theories, and declares itself in- 

 capable of conceiving them. Their inconceivableness to him does not, how- 

 ever, result from any thing in the theories themselves, intrinsically and a 

 priori repugnant to the human faculties ; it results from the repugnance 

 between them and a portion of the facts ; which facts as long as he did 

 not know, or did not distinctly realize in his mental representations, the 

 false theory did not appear other than conceivable ; it becomes inconceiv- 

 able, merely from the fact that contradictory elements can not be combined 

 in the same conception. Although, then, his real reason for rejecting theo- 

 ries at variance with the true one, is no other than that they clash with his 

 experience, he easily falls into the belief, that he rejects them because they 

 are inconceivable, and that he adopts the true theory because it is self-evi- 

 dent, and does not need the evidence of experience at all. 



* Phil, of Disc, p. 338. t Ibid., p. 463. 



