152 FRAGMENTS OF SCIENCE. 



assuming it to diminish simply as the distance, and of 

 making it purely terrestrial. He knew the relation 

 between the velocities, spaces, and times of falling 

 bodies, and had distinct ideas of capillary attraction. 

 He improved the hydrometer. The determinations of 

 the densities of bodies, as given by Alhazen, approach 

 very closely to our own. * I join,' says Draper, ' in the 

 pious prayer of Alhazen, that in the day of judgment 

 the All-Merciful will take pity on the soul of'Abur- 

 Raihan, because he was the first of the race of men to 

 construct a table of specific gravities.' If all this be 

 historic truth (and I have entire confidence in Dr. 

 Draper), well may he * deplore the systematic manner 

 in which the literature of Europe has contrived to put 

 out of sight our scientific obligations to the Mahom- 

 medans.' 1 



The strain upon the mind during the stationary 

 period towards ultra-terrestrial things, to the neglect of 

 problems close at hand, was sure to provoke reaction. 

 But the reaction was gradual ; for the ground was dan- 

 gerous, and a power was at hand competent to crush the 

 critic who went too far. To elude this power, and still 

 allow opportunity for the expression of opinion, the 

 doctrine of ' two-fold truth ' was invented, according to 

 which an opinion might be held c theologically,' and the 

 opposite opinion ' philosophically.' 2 Thus, in the thir- 

 teenth century, the creation of the world in six days, and 

 the unchangeableness of the individual soul, which had 

 been so distinctly affirmed by St. Thomas Aquinas, 

 were both denied philosophically, but admitted to be 

 true as articles of the Catholic faith. When Protagoras 

 uttered the maxim which brought upon him so much 

 vituperation, that ' opposite assertions are equally true,' 



1 ' Intellectual Development of Europe/ p. 359. 

 'Lange,' 2nd edit. pp. 181, 182. 



