IJN" THE LAST HALF-CENTUEY. 67 



in virtue of which, any two such, parti- 

 cles, without any external impression of 

 motion, or intermediate material agent, 

 were supposed to tend to approach or re- 

 move from one another ; and this view of 

 the duality of the causes of motion is 

 very widely held at the present day. 



Another important result of investiga- 

 tion, attained in the seventeenth century, 

 was the proof and quantitative estimation 

 of physical inertia. In the old philoso- 

 phy, a curious conjunction of ethical and 

 physical prejudices had led to the notion 

 that there was something ethically bad 

 and physically obstructive about matter. 

 Aristotle attributes all irregularities and 

 apparent dysteleologies in nature to the 

 disobedience, or sluggish yielding, of mat- 

 ter to the shaping and guidiug influence 

 of those reasons and causes which were 

 hypostatised in his ideal 'Forms.' In 

 modern science, the conception of the 

 inertia, or resistance to change, of matter 



