PHILOSOPHY OF NATURE LEIBNITZ'S IDEAS. 55 



ent activities, explainable neither by geometry nor by 

 mechanics. 



Let us last question the physical science of our own 

 day, and we shall discover in it still the same ideas. It re- 

 duces every thing to vibrations, both those of what it terms 

 material atoms, and those of what it calls ether. In its 

 view, physical phenomena are explained by the system of 

 motions of atoms and of the ether, and, since these motions 

 may be transmuted into one another according to a mathe- 

 matical law, it follows that relations of equivalence exist 

 among the various manifestations of physical activity 

 that there is such a thing, for instance, as a mechanical 

 equivalent of heat, a calorific equivalent of electricity, etc. 

 Now, that internal motion revealed by analysis and induc- 

 tion, that corpuscular agitation which gives bodies the qual- 

 ities without which they could not be perceived, namely, 

 weight, color, heat, etc. that motion, under every form, 

 implies a moving principle, something simple and irredu- 

 cible, a spontaneity similar to that Leibnitz conceives of in 

 monads. What is that living force, that potential energy, 

 that virtual energy, which physicists so often employ in 

 their speculations, if it is not the same thing as metaphys- 

 ical actualities, the intelligible cause of acts of force, of 

 tendencies like those the soul feels within itself? Will it 

 be said that all these manifold and varying aspects of phys- 

 ical force are derivative from the sheer mechanical force, 

 whose sum in the universe is unchanging ? But, then, why 

 does motion become at one place heat, at another light, 

 and still at another electricity ? Must it not be because, 

 besides those monads that are the spring of motion, others 

 exist, whose special function, from the point of view of our 

 sensibility, is to act on different perceptive capacities from 

 those by which we cognize motiofi ? 



Under another aspect we recognize in our sciences of 

 to-day some of the great thoughts of Leibnitz, thanks to 



