46 SCIENCE AND THE HUMAN MIND 



evidence, he was able to obtain a general acceptance 

 of his views. 



As an example of Aristotle's method, his treatment 

 of the problem of falling bodies is instructive. Demo- 

 critus had taught that in a vacuum all bodies would 

 fall at an equal rate, and that the observed differences 

 were due to the unequal resistance of the air. This 

 opinion is correct, though Democritus had no experi- 

 mental evidence to bring forward. Aristotle accepts 

 the statement that in a vacuum bodies would fall 

 equally fast, but argues that such a conclusion is 

 inconceivable, and that therefore there can never be 

 a vacuum. With the possibility of empty space he 

 rejects all the allied concepts of the atomic theory. 

 If all substances were composed of the same ultimate 

 material, Aristotle argued that they would all be 

 heavy by nature, and nothing would be light in itself 

 or tend to rise spontaneously. A large mass of air 

 or fire would then be heavier than a small mass of 

 earth or water, and the earth or water could not sink 

 through air or fire as it is known to do. 



Aristotle's error arose from the fact that, in common 

 with other philosophers of his age, he had no notion 

 of the conceptions now known as density or specific 

 gravity ; he failed to see that it is the weight per unit 

 volume which is the determining factor in questions 

 of rise and fall, and attributed the motion to an innate 

 instinct leading everything to seek its natural resting- 

 place. This doctrine that bodies are essentially heavy 

 or light in themselves was accepted with the rest of 

 Aristotle's philosophy by the schoolmen and theo- 

 logians of the later Middle Ages, and his dead hand 

 held back the flowing tide of knowledge till Galileo, 



