Page:Philosophical Review Volume 19.djvu/259

245 it is sometimes convenient to ignore our conceptions of force, and to deal with its effects by means of mathematical symbols. This treatment will not suffice for philosophy, which demands a more fundamental search for first principles.

This primary basis of our ideas, Spencer found in the study of psychology. He pointed out that our ideas of matter and motion are largely complexes made up from our muscular sensations of pressure and tension. That which affects our muscular sensations actively we call force; and all our conceptions, visual or tactual, are ultimately compounded from this primary element. That which affects our muscular sensations passively, we call matter; and this element of passive force (if we may use the term) or inertia, is the ineradicable element of our ideas of body. As both matter and energy are known to us in terms of force, the phrase, persistence of force, includes both the modified conception of the conservation of energy and the indestructibility of matter.

Without commenting fully on this psychological analysis, it is at once clear that Spencer's principle is more fundamental than the concept of modern physics known as the conservation of energy. It is not necessary to lay any particular stress on this point, because we shall find that Spencer, in formulating his law of evolution, returns from his ultimate analysis to the more familiar ground of matter and motion. In still another way does this conception differ from the corresponding chemical and physical principles as commonly expressed. Considered philosophically, it is not merely a valuable inductive principle, but a necessary truth of the same order as our fundamental conceptions of space and time. Although the formulation of these truths has resulted from modern physical and chemical investigations, yet it remained for Spencer to point out that they are like axioms in that proof of them is impossible.