FORCE AND INTERACTION. 73 



are now discarded by science. We now say that 

 a force Is one half a stress, and substitute interact- 

 ion for the distinction of active and passive ; and 

 indeed the fact that action^ and reaction are equal 

 and opposite has become as obvious a necessity of 

 thought as it ever was to the Greeks, that one 

 thing must be acted upon and the other act upon it. 



And yet what business have we to speak even of 

 interaction ? All we see is how two bodies seem to 

 change each other's motions, without being able to 

 grasp kow they do so in their action at a distance. 

 Even so w^e have assumed too much ; for what right 

 have we to assume that one influences the other, 

 what justification for defining force as the cause of 

 motion, for applying our conception of causation to 

 the thino^s around us ? 



\ II. Since the time of Hume the vital import- 

 ance to science of the conception of causation has 

 been fully recognised, and it would now be generally 

 admitted that a successful assault upon it is in itself 

 sufficient to establish the case of Scepticism. And 

 fully proportionate to its importance are the diffic- 

 ulties of justifying this principle. Its historical 

 antecedents are In themselves almost sufficient to 

 condemn It ; and the existing divergences as to its 

 nature make a consistent defence almost impossible. 



Originally, as has been remarked, the conception 

 of cause was a. transference of the internal sense of 

 volition and effort to things outside the organism. 

 The changes in the world were supposed to be due 

 to the action of immanent spirits. In course of 

 time these divine spirits were no longer regarded 

 as directly causing events, but as being the first 

 causes which set secondary causes in motion. It 

 was then supposed that cause and effect were con- 



