474 Fundamental Ideas of Matter and Force. 



considerable portion of the Principiais devoted to its refutation. 

 It is not a little singular that the isotropic hypothesis has the 

 same hold upon modern physical science, with, as it seems to me, 

 just as little reason, having nothing but arbitrary conception to 

 rest upon, and being itself as difficult to explain as the facts for 

 the explanation of which it was invented. 



Another obstacle to the reception of the a priori method of 

 philosophy I have been discussing is the practice which has been 

 very prevalent of giving to a general law the name of a principle, 

 and employing it deductively. The law of the conservation of 

 vis viva is expressible by a formula, which is obtained by mathe- 

 matical reasoning from established dynamical principles. It is 

 therefore strictly a law, being reached by the only process that 

 is proper for demonstrating laws, besides that of direct experi- 

 ment. And yet the principle of the conservation of vis viva is 

 frequently spoken of, and attempts have been made to construct 

 upon it a general physical system. The attempts I refer to have 

 not been successful, and have only stood in the way of a sounder 

 philosophical method. 



I have now accomplished my purpose of making a final effort 

 to gain attention to views long entertained and often urged re- 

 specting the course that ought to be pursued in theoretical 

 physics. There is, I conceive, at the present time no greater 

 scientific need than to ascertain what is the true method of 

 philosophy. It seems to be admitted on all sides that the ex- 

 perimental demonstration of facts and laws, although a neces- 

 sary part of philosophy, does not constitute the whole of it. 

 The efforts that are continually made, both by experimenters 

 and theoretical calculators, to give reasons for the results of ex- 

 periments is a kind of evidence that theory is felt to be neces- 

 sary for completing scientific knowledge. But it must be con- 

 fessed that the prevailing theoretical speculations present a 

 wonderful diversity of views and hypotheses, and seem to be 

 guided by no definite rules or principles, the faculty of imagining 

 having a large share in framing them. In the greater number 

 of the speculations the hypotheses which they involve are far 

 from satisfying the three conditions I have insisted upon in this 

 communication. The acceptance of those conditions is a neces- 

 sary step in modern philosophy. I beg permission to add that, 

 having after much consideration adopted hypotheses which 

 satisfy the requisite conditions, and having to a very considerable 

 extent tested their truth by the results of mathematical calcula- 

 tion, I have some reason to say that these hypotheses are the 

 necessary foundations of theoretical research, and that I have 

 indicated the true method of philosophy. 



Cambridge, May 19, 1866. 



