II THE PROGRESS OF SCIENCE 83 



on another at a distance through a vacuum, the 

 ultimate particles of matter were generally 

 assumed to be the seats of perennial causes of 

 motion termed " attractive and repulsive forces," 

 in virtue of which, any two such particles, with- 

 out any external impression of motion, or inter- 

 mediate material agent, were supposed to tend to 

 approach or remove 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 investigation, at- 

 tained in the seventeenth century, was the proof 

 and quantitative estimation of physical inertia. In 

 the old philosophy, 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 

 ah 1 irregularities and apparent dysteleologies in 

 nature to the disobedience, or sluggish yielding, 

 of matter to the shaping and guiding influence of 

 those reasons and causes which were hypostatised 

 in his ideal " Forms." In modern science, the con- 

 ception of the inertia, or resistance to change, of 

 matter is complex. In part, it contains a corollary 

 from the law of causation : A body cannot change 

 its state in respect of rest or motion without a 

 sufficient cause. But, in part, it contains general- 

 isations from experience. One of these is that 

 there is no such sufficient cause resident in any 

 body, and that therefore it will rest, or continue 



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