258 THE DIRECTION OF MOTION. 



Or the motion may itself be in part metamorphosed into 

 some obstructive form of force ; as in metals, the conducting 

 power of which is, for the time, decreased by the heat which 

 the passage of electricity itself generates. The real ques- 

 tion is, however, what structural modification, if any, is pro- 

 duced throughout the matter traversed, apart from inci- 

 dental disturbing forces — apart from everything but the 

 necessary resistance of the matter: that, namely, which re- 

 sults from the inertia of its units. If we confine our 

 attention to that part of the motion which, escaping trans- 

 formation, continues its course, then it is a corollary from 

 the persistence of force that as much of this remaining mo- 

 tion as is taken up in changing the positions of the units, 

 must leave these by so much less able to obstruct subsequent 

 motion in the same direction. 



Thus in all the changes heretofore and at present dis- 

 played by the Solar System; in all those that have gone on 

 and are still going on in the Earth's crust; in all processes of 

 organic development and function ; in all mental actions and 

 the effects they work on the body; and in all modifications 

 of structure and activity in societies ; the implied movements 

 are of necessity determined in the manner above set forth. 

 Wherever we see motion, its direction must be that of the 

 greatest force. Wherever we see the greatest force to be 

 acting in a given direction, in that direction motion must 

 ensue. These are not truths holding only of one class, or of 

 some classes, of phenomena; but they are among those 

 universal truths by which our knowledge of phenomena in 

 general is unified. 



