THALES TO LUCRETIUS 



to retain the old Ionic doctrine of a substance that 

 " knows " all things, and to identify this with the 

 new theory of a substance that " moves " all things.' 

 Thus far speculation has run largely on the 

 origin of life -forms, but now we find revival of 

 speculation about the nature of things generally, 

 and the formulation of a theory which links Greek 

 cosmology with early nineteenth-century science with 

 Dalton's ATOMIC THEORY. Democritus of Abdera, 

 who was born about 460 B.C., has the credit of having 

 elaborated an atomic theory, but probably he only 

 further developed what Leucippus had taught be- 

 fore him. Of this last-named philosopher nothing 

 whatever is known ; indeed, his existence has been 

 doubted, but it counts for something that Aristotle 

 gives him the credit of the discovery, and that Theo- 

 phrastus, in the first book of his Opinions, wrote of 

 Leucippus as follows : * He assumed innumerable 

 and ever-moving elements, namely, the atoms. And 

 he made their forms infinite in number, since there 

 was no reason why they should be of one kind 

 rather than another, and because he saw that there 

 was unceasing becoming and change in things. He 

 held, further, that what is is no more real than what 

 is not, and that both are alike causes of the things 

 that come into being ; for he laid down that the 

 substance of the atoms was compact and full, and 

 he called them what ts, while they moved in the 

 void which he called what is not, but affirmed to be 

 just as real as what is! Thus did * he answer the 

 question that Thales had been the first to ask.' 



Postponing further reference to this theory until 

 the great name of Lucretius, its Roman exponent, is 



