260 PHYSICAL SCIENCE IN THE MIDDLE AGE.S. 



were self-evident ; and when, at a later period, ex- 

 perimental philosophers, such as Galileo and Boyle, 

 ventured to contradict these current maxims, their 

 new principles sounded in men's ears as strange 

 as they now sound familiar. Thus Boyle promul- 

 gated his opinions on the mechanics of fluids, as 

 " Hydrostatical Paradoxes, proved and illustrated 

 by experiments." And the opinions which he there 

 opposes, are those which the Aristotelian philo- 

 sophers habitually propounded as certain and indis- 

 putable ; such, for instance, as that " in fluids the 

 upper parts do not gravitate on the lower;" that 

 "a lighter fluid will not gravitate on a heavier;" 

 that " levity is a positive quality of bodies as well 

 as gravity." So long as these assertions were left 

 uncontested and untried, men heard and repeated 

 them, without perceiving the incongruities which 

 they involved : and thus they long evaded refu- 

 tation, amid the vague notions and undoubting 

 habits of the stationary period. But when the 

 controversies of Galileo's time had made men think 

 with more acuteness and steadiness, it was dis- 

 covered that many of these doctrines were incon- 

 sistent with themselves, as well as with experiment. 

 We have an example of the confusion of thought 

 to which the Aristotelians were liable, in their 

 doctrine concerning falling bodies. "Heavy bodies," 

 said they, " must fall quicker than light ones ; for 

 weight is the cause of their fall, and the weight 

 of the greater bodies is greater." They did not 



