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Rh, or the concurrence of antecedents, on which it is invariably and unconditionally consequent." I pointed out to my friend that, when this assemblage of conditions is analyzed, it is uniformly found resolvable into two categories, which may be distinguished as the dynamical and the material; the former supplying the force or power to which the change must be attributed, while the latter affords the conditions under which that power is exerted. Thus, I urged, when a man falls from a ladder because (as is commonly said) of the breaking of the rung on which his foot was resting, the real or dynamical cause of his fall is the force of gravity, or attraction of the earth, which pulls him to the ground when his foot is no longer supported; the loss of support being only the material condition or collocation, which allowed the force previously acting as pressure on the rung to produce the downward motion of the man who stood upon it.

To this Mr. Mill's reply was, that the distinction is one of metaphysics, not of logic. I ventured, however, to press on him that, to whichever department of philosophy this point is to be referred, it is one of fundamental importance; that, assuming experience as the basis of our knowledge, we recognize the downward tendency of every body heavier than air, by our sense of muscular tension in lifting it from the ground, or in resisting its descent toward the earth; and that our cognition of force through this form of sensation, being thus quite as immediate and direct as our cognition of motion through the visual sense, ought to be equally taken account of.

The promulgation, about the same time, of the doctrine of the "Correlation of the Physical Forces" by Professor (now Sir William) Grove, and the researches of Mr. Joule on the "Mechanical Equivalent of Heat," seemed to me to bring this view of dynamical causation into yet greater importance, by showing that what is true of that form of force which produces or resists mechanical (or what is now distinguished as molar) motion, may be legitimately extended to those other forms which are manifested in the molecular changes that express themselves in chemical action, or impress us with the sensations of heat, light, etc. Partaking of the general ignorance at that time prevalent in this country of the doctrine of "Conservation of Energy," already promulgated in Germany by Mayer and Helmholtz, I myself endeavored to carry Professor Grove's principle into the domain of biology, by showing that what physiologists had been accustomed to call vital force may be regarded as having the same "correlation" with the various forms of physical force as they have with each other. And in the introduction to the fourth edition of my "Human Physiology" (published in 1853) I thus explicitly defined my position:

When this assemblage of antecedents is analyzed, it is uniformly found that they may be resolved into two categories, which may be distinguished as the