66o THE POPULAR SCIENCE MONTHLY. 



eral 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 

 were grave and numerous. He affirmed that only in man we Jiad 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, not at the front, but at the back of every man's 

 head. 



There is one essential quality in physical conceptions w^hich was 

 entirely wanting in those of Aristotle and his followers. I wish it 

 could be expressed by a word untainted by its associations ; it signi- 

 fies a capability of being placed as a coherent picture before the mind. 

 The Germans express the act of picturing by the word vorstellen^ and 

 the picture they call a Yorstellung, We have no word in English 

 which comes nearer to our requirements than iraagination^ and, taken 

 with its proper limitations, the word answers very well ; but, as just 

 intimated, it is tainted by its associations, and therefore objectionable 

 to some minds. Compare, with reference to this capacity of mental pres- 

 entation, the case of the Aristotelian, w^ho refers the ascent of water 

 in a pump to Nature's abhorrence 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 fall and rise of the barometer being 

 clearly figured as the balancing of two varying and opposing press- 

 ures. 



During the drought of the middle ages in Christendom, the Ara- 

 bian intellect, as forcibly shown by Draper, was active. With the 

 intrusion of the Moors into Spain, cleanliness, order, learning, and re- 

 finement, took the place of their opposites. When smitten w4th the 

 disease, the Christian peasant resorted to a shrine ; the Moorish one 

 to an instructed physician. The Arabs encouraged translations from 

 the Greek philosophers, but not from the Greek poets. They turned 

 in disgust " from the lewdness of our classical mythology, and de- 

 nounced as an unpardonable blasphemy all connection between the 

 impure Olympian Jove and the Most High God." Draper traces still 

 further than Whewell the Arab elements in our scientific terms, and 

 points out that the under-garment of ladies retains to this hour its 

 Arab name. He gives examples of what Arabian men of science ac- 

 complished, dwelling particularly on Alhazen, who was the first to 

 correct the Platonic notion that rays of light are emitted by the eye. 

 He discovered atmospheric refraction, and points out that we see the 

 sun and moon after they have set. He explains the enlargement of 

 the sun and moon, and the shortening of the vertical diameters of both 



