and the action of the forces upon the material particles is likewise an ultimate empirical datum, and therefore inexplicable. Force and matter, though presupposing each other in action, are fundamentally disparate; they are essentially distinct, and mutually irreducible entities. Matter, as such, is passive, dead; all motion or life is caused by force; and the only possible solution of the problems of physiology, no less than of physics and chemistry, consists in the enumeration of the forces acting upon the material particles, and in the exact quantitative determination of the effects produced by their action.
This statement of the tenets of the prevailing physical philosophy, to be exact, requires at most two qualifications. In the first place, the recent doctrine of the correlation and mutual convertibility of the physical forces, as a part of the theory of the conservation of energy, has shaken, if not destroyed, the conception of a multiplicity of independent original forces. And, in the second place, physiologists, like Du Bois-Reymond, recognize force as the invariable concomitant, if not the essential attribute of matter, and assume that to every constant primordial mass belongs a constant primordial quantity of force, so that the problem of physics, chemistry, and physiology, resolves itself into the quantitative determination of the mechanical interactions of material constants primarily endowed with forces acting equally in all directions, or, as they express it, constant central forces.
I have endeavored, thus far, to show that there are no absolute constants of mass; that both the hypothesis of corpuscular "atoms" and that of "centres of force" are growths of a confusion of the intellect which mistakes conceptual elements of matter for real elements; that these elements—force and mass, or force and inertia—are not only inseparable, as is conceded by the more thoughtful among modern physicists (or, as they usually, but inaccurately express it, that there is no force without matter, and no matter without force), but that neither of these elements has any reality as such, each of them being simply the conceptual correlate of the other, and thus the condition both of its realization in thought and of its objectivation to sense. The tendency to deal with these elements as separate and separately real entities is so irrepressible, however, that it is necessary to subject them to still further discussion, in order to clear up the prevalent confusion in regard to them.
Newton's original definition of inertia was in terms of force. According to him ("Principia," Definitio III.), "there is inherent in matter a force, a power of resistance, in virtue of which every body, as far as in it lies, perseveres in a state of rest, or of uniform rectilinear motion." In the definitions since Newton's time, the term "force" has usually been avoided. Thus Young ("Mechanics," p. 117) defines inertia as "the incapability of matter of altering the state into which it is put by any external cause, whether that state be rest or motion;" and similarly Whewell ("Mechanics," p. 245), as "the quantity of matter