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namical and the material, the former supplying the force or power to which the change must be attributed, while the hitter afford the conditions under which that power is exerted. Thus, in a steam-engine we see the dynamical agency of heat made to produce mechanical power by the mode in which it is applied: first, to impart a mutual repulsion to the particles of water; and then, by means of that mutual repulsion, to give motion to the various solid parts of which the machine is composed. And thus, if asked what is the cause of the movement of the steam-engine, we distinguish in our reply between the dynamical condition supplied by the heat and the material condition (or assemblage of conditions) afforded by the "collocation" of the boiler, cylinder, piston, valves, etc. . . . In like manner, if we inquire into the cause of the germination of a seed—which has been brought to the surface of the earth after remaining dormant through having been buried deep beneath the soil for (it may be) thousands of years—we are told that the phenomenon depends upon warmth, moisture, and oxygen; but out of these we single warmth as the dynamical condition, while the oxygen and the water, with the organized structure of the seed itself, and the organic compounds which are stored up in its substance, constitute the material.

The subsequent general recognition by the scientific world of the "correlation" between the forces of nature (under whatever form expressed) has thus given a breadth of foundation to the dynamical doctrine of causation which it previously lacked; and the doctrine, having been afterward formally developed by Professor Bain, was summarized by J. S. Mill, in the later editions of his "Logic," almost in the very terms in which I had originally propounded it to him in conversation, and had publicly expressed it in the extract just cited: "The chief practical conclusion drawn by Professor Bain, bearing on causation, is that we must distinguish, in the assemblage of conditions which constitutes the cause of a phenomenon, two elements: one, the presence of a force; the other, the collocation or position of objects which is required in order that the force may undergo the particular transmutation which constitutes the phenomenon."^[1] Mr. Mill himself still preferred, however, to express the principle in terms of motion rather than in terms of force: "If the effect, or any part of the effect, to be accounted for consists in putting matter in motion, then any of the objects present which has lost motion has contributed to the effect; and this is the true meaning of the proposition that the cause is that one of the antecedents which exerts active force." As this mode of expressing the facts is sanctioned by high authorities at the present time, it may be well for me to explain more fully the basis of my original contention, that our cognition of force is quite as immediate and direct as our cognition of motion; in fact (as I think I shall be able to prove), even more fundamental, inasmuch as our cognition of matter itself is in great degree dependent upon it.

It has been recently well said that "all true science involves both the knowledge of nature and the knowledge of man; it includes the study of mind, as well as of matter. A philosopher may pursue either,

1. "System of Logic," eighth edition, vol. i., p. 406.