THE FORCE BEHIND NATURE. 615 



cedent, or the concurrence of antecedents, on whicli 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 

 dlinamical and the material ; the former supplying the /orce ox power 

 to which the change must be attributed, w^hile 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 matericd condition or collocation, which al- 

 lowed the force previously acting as pressure on the rung to prodvtce 

 the downward motion of the man who stood upon it. 



To this Mr. Mill's reply was, that the distinction is one of meta- 

 physics, 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 oi 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 

 " CoiTelation 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 dynamiccd 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 distin- 

 guished as molar) motion, may be legitimately extended to those other 

 forms which are manifested in the molecidar 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 Physi- 

 ology " (published in 1853) I thus exj^licitly 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 dy- 



* " On the Mutual Relations of the Vital and Physical Forces," in " Philosophical 

 Transactions," 1850. 



