436 JAMES CLERK MAXWELL. [CHAP. XIV. 



likely to espouse doctrines which harmonise with the par- 

 ticular set of ideas to which he is induced, by the process of 

 education, to confine his attention. What will be the 

 probable effect if these ideas happen mainly to be those of 

 modern physical science ? 



The intimate connexion between physical and meta- 

 physical science is indicated even by their names. What 

 are the chief requisites of a physical laboratory ? Facilities 

 for measuring space, time, and mass. What is the occupa- 

 tion of a metaphysician ? Speculating on the modes of 

 difference of co-existent things, on invariable sequences, 

 and on the existence of matter. 



He is nothing but a physicist disarmed of all his 

 weapons, a disembodied spirit trying to measure distances in 

 terms of his own cubit, to form a chronology in which inter- 

 vals of time are measured by the number of thoughts which 

 they include, and to evolve a standard pound out of his own 

 self-consciousness. Taking metaphysicians singly, we find 

 again that as is their physics, so is their metaphysics. 

 Descartes, with his perfect insight into geometrical truth, 

 and his wonderful ingenuity in the imagination of mechanical 

 contrivances, was far behind the other great men of his time 

 with respect to the conception of matter as a receptacle of 

 momentum and energy. His doctrine of the collision of 

 bodies is ludicrously absurd. He admits, indeed, that the 

 facts are against him, but explains them as the result either 

 of the want of perfect hardness in the bodies, or of the 

 action of the surrounding air. His inability to form that 

 notion which we now call force is exemplified in his ex- 

 planation of the hardness of bodies as the result of the 

 quiescence of their parts. 



" Neque profecto ullum glutinum possumus excogitare, 

 quod particulas durorum corporum firmius inter se conjungat, 

 quam ipsarum quies." Princip., Pars II. LV. 



Descartes, in fact, was a firm believer that matter has 

 but one essential property, namely extension, and his in- 

 fluence in preserving this pernicious heresy in existence 

 extends even to very recent times. Spinoza's idea of matter, 



