150 FKAGMENTS OF SCIENCE. 



He affirmed that a vacuum could not exist, and proved 

 that if it did motion in it would be impossible. He 

 determined a priori how many species of animals must 

 exist, and showed on general principles why animals 

 must have such and such parts. When an eminent 

 contemporary philosopher, who is far removed from 

 errors of this kind, remembers these abuses of the 

 a priori method, he will be able to make allowance for 

 the jealousy of physicists as to the acceptance of so- 

 called a priori truths. Aristotle's errors of detail, as 

 shown by Eucken and Lange, were grave and numerous. 

 He affirmed that only in man we had the beating of the 

 heart, that the left side of the body was colder than the 

 right, that men have more teeth than women, and that 

 there is an empty space at the back of every man's head. 

 There is one essential quality in physical concep- 

 tions, which was entirely wanting in those of Aristotle 

 and his followers a capability of being placed as 

 coherent pictures before the mind. The Germans 

 express the act of picturing by the word vorstellen, and 

 the picture they call a Vorstellung. We have no word 

 in English which comes nearer to our requirements 

 than Imagination ; and, taken with its proper limita- 

 tions, the word answers very well. But it is tainted by 

 its associations, and therefore objectionable to some 

 minds. Compare, with reference to this capacity of 

 mental presentation, the case of the Aristotelian, who 

 refers the ascent of water in a pump to Nature's abhor- 

 rence of a vacuum, with that of Pascal when he proposed 

 to solve the question of atmospheric pressure by the 

 ascent of the Puy de Dome. In the one case the terms 

 of the explanation refuse to fall into place as a physical 

 image ; in the other the image is distinct, the descent and 

 rise of the barometer being clearly figured beforehand 

 as the balancing of two varying and opposing pressures, 



