6i6 THE POPULAR SCIENCE MONTHLY. 



namical and the material^ the former supplying the force or poicer to which the 

 change must be attributed, while the latter 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, w^e 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 ex- 

 pressed) has thus given a breadth of foundation to the dynamical doc- 

 trine 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 con- 

 stitutes 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 transmu- 

 tation which constitutes the phenomenon." * 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, 

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



