PHILOSOPHY OF NATURE—LEIBNITZ’S IDEAS. 5D 
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, ete. 
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 motion ? 
Under another aspect we recognize in our sciences of 
to-day some of the great thoughts of Leibnitz, thanks to 
