150 FRAGMENTS OF SCIENCE. 



or worse, calm or violent no real mechanical con- 

 ception regarding it lying at the bottom of his mind. 

 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 allowances 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 numer- 

 ous. He affirmed that only in man we had the beating 

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

 colder that 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 essent'al quality in physical concep- 

 tions, which was entirely wanting in those of Aristotle 

 and his followers a capability of being placed as co- 

 herent pictures before the mind. The Germans ex- 

 press 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 pro- 

 posed 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 



