1885.] on hoiv Thought presents itself in Nature. 187 



trivance by which they take notice of the fact that in every such case 

 the drifting motion of both the bodies is affected, both of the body 

 acting on the stone and of the stone. What is called the mass of the 

 stone is really the measure of the acceleration received by the other 

 body while it and the stone are acting on each other, so that the 

 ratio of t]ie two masses is the ratio of the accelerations taken in 

 reverse order ; or if we wish to make the statement general, the mass 

 of the stone is the measure of all the changes of drifting motions 

 going on elsewhere throughout the material universe which have 

 relation to the simultaneously existing motions which we call the 

 stone. The real cause that is operating is the proximity of the 

 molecular motions in the stone to the molecular motions of some other 

 body ; one part of the effect is a change in the molar motions of both 

 bodies, i. e. in the drifting about of their masses of molecular motions. 

 Other effects also arise, but this is the part of the effects which the 

 science of dynamics investigates. In fact dynamics does not concern 

 itself in the least with the motions going on within the molecules of 

 the bodies with which it is dealing. It is the science of the transfer- 

 ence from place to place of underlying motions. The symbols for 

 force and mass express vaguely the presence of adequate causes, while 

 in terms of the effects these symbols are, in the science of dynamics, 

 quite definite. 



The proper description of the law of gravitation is that towards 

 each particle of ponderable matter, i. e. towards the molecular motions 

 within a small compact volume, all the other molecular motions that are 

 going on in the universe are being drifted with accelerations that are 

 inversely as the squares of their distances from that centre ; and the 

 ratio of the acceleration thus impressed by one particle on another 

 when comj)ared with the amount received by the first from the 

 second is that ratio which is called the ratio of their masses. The . 

 resultants of these accelerations may properly be called things, in 

 the sense that we find them really existing in nature : all the rest 

 are contrivances of the mind to enable it sufficiently to grasn the 

 phenomena. 



Of Substance. 



Another prevalent impression is that, in addition to the motions 

 we find existing in nature, there is a mysterious entity which is 

 spoken of under the name of substance. Now Science has never 

 found, and therefore knows nothing of, any object devoid of internal 

 motions such as that often supposed under this name, added to 

 external nature by the mind, not found by it in nature. Whence, 

 then, comes it that there is an almost universal persuasion that under- 

 lying every motion there must be some thing to be moved ? We need 

 not search far to find the answer to this question. It has arisen from 

 experience, from abundant experience : from our experience and the 

 experience of the whole series of our progenitors down from the dawn 

 of such organised thought upon the earth as we possess. Neither we 

 nor any member of this long series ever felt or saw a primary motion 



