34 THE FOUNDATIONS OF SCIENCE 



Although the concept of mass was only formu- 

 lated clearly by Newton, the problem of matter, 

 the most characteristic property of which is mass, 

 is at least as old as the Greek philosophers. The 

 influence of this age-long familiarity is evident in 

 Newton's handling of the subject ; for, instead of 

 defining mass in terms of force, he defined it as " the 

 quantity of matter in a body," and treated force 

 as a derived unit. But, in truth, matter is best 

 known to us through its mass, though historically 

 matter itself seems the more familiar idea. 



Even among the Greeks two views of the nature 

 of matter were held. One theory taught that it 

 was continuous and infinitely divisible into similar 

 parts, so that water was water however far the sub- 

 division went. But Leucippus and Democritus put 

 forward the opposite view that, at a certain stage, 

 division could go no further, and that there ultimate 

 particles or atoms would be reached. On this 

 atomic theory, the different properties of various 

 substances were due to differences in the size, struc- 

 ture, and arrangement of particles of the same 

 ultimate nature. 



But this speculation was in advance of the age, 

 and no facts or experiments could then be adduced 

 in its verification. Hence, when it was criticised by 

 Aristotle in the light of his own preconceptions, it 

 withered away. It was revived by the Roman 

 poet Lucretius, but Aristotle's influence alone sur- 



