AKISTOTLE'S METHOD. 27 



upwards and downwards, and so the upward and downward 

 motions natural to bodies could not take place.* Again, if 

 bodies of similar shapes pass through a medium, such as air 

 or water, then those which have a greater driving-force 

 (poTiri) — due to their heaviness, in the case of bodies to which a 

 downward motion is natural, or to their lightness, in the case 

 of bodies to which an upward motion is natural — will move 

 more quickly through the same distance. This ought to 

 happen also when the bodies pass through a void, but 

 this is impossible, for vv^hat reason is there for the swifter 

 motion ? In water or other medium this happens, of 

 course, since the heavier bodies, e.g., divide the medium 

 more quickly by reason of their greater heaviness. A 

 body in motion divides the medium by reason of its shape 

 or its driving force {po'^y}), and, when there is no medium, 

 all bodies ought to move with equal velocities, but this 

 is impossible.! Having thus argued, he says that it is 

 clear therefore that there cannot be a separate void 

 (}csx,(^pia-ixevov KEv6y).l Without attempting to analyse the 

 above arguments any further, it will be evident that the 

 introduction of ideas, such as, for instance, that it is 

 necessary to distinguish upward and downward directions 

 before it can be said that motion is possible, that bodies have 

 certain motions natural to them, and that the velocity of a 

 body depends on its shape and on its heaviness or lightness, 

 qualities considered to be inherent in the body, make it 

 impossible to come to any correct conclusions. 



Finally, Aristotle's conclusion that the blood of the right 

 chamber of the heart and of the right side of the body is 

 hotter than that of the left chamber or side may have been 

 based, in part, on observations, for he was aware of differences 

 of consistency, turbidity, and temperature in the blood from 

 different parts of the same animal. Observations were not 

 relied on, however, to any important extent in this instance. 

 His arguments in P. A. ii. c. 2, 648a show clearly that his 

 conclusion that the blood of the right chamber of the heart 

 and of the right side of the body is hotter than that of the 

 left chamber or side followed from his idea that the right 

 is nobler or more honourable than the left. This idea, it 

 will be noticed, has no necessary connection with the ques- 

 tion of differences of temperature of the blood in different 

 parts of the body. 



■'' Physics, iv. c. 8, ss. 3 and 4. -j Physics, iv. c. 8, ss. 11 and 12. 



I Physics, iv. c. 8, s. 16. 



