The Background of Modern Science 11 



it sounds when put in this extreme way. It grew out of 

 the everyday observation that hght objects Uke pillows 

 or feathers fall rather slowly, while heavy ones like stones 

 or cannon balls fall fast. Aristotle built this idea into his 

 system of explanation which relied heavily on the idea 

 that objects have a series of intrinsic properties and pur- 

 poses which largely determine their behavior. 



What Galileo essentially did was to show that it was 

 much easier to understand motion if one looked at it as 

 something imparted to an object by outside forces. He 

 never quite said it in these words, but his whole line of 

 thinking and experimenting led in this direction and paved 

 the way for Newton's very clear statement of the case later 

 on. By conducting his experiments in such a way that 

 forces attributable to friction and air resistance could be 

 neglected, he was able to show that, in fact, all bodies 

 accelerate at equal rates when rolling down an inclined 

 plane or falling freely. Later investigators invented the 

 air pump and confirmed these views by repeating the 

 experiments in a vacuum so as to eliminate air resistance 

 completely. In summary then, Galileo, by a combination 

 of rare insight and uncommon abiUty to design what we 

 now call controlled experiments, was able to demonstrate 

 the basic principles of motion. These principles made it 

 easy to describe the behavior of any object in terms of 

 outside forces acting upon it, and paved the way for what 

 is now rather loosely called the mechanistic way of look- 

 ing at the universe in general. 



GaHleo's work had far-reaching effects. In the first place, 

 it turned the attention of thoughtful men to the importance 

 of checking the results of reason and revelation by an 

 appeal to the observed facts under carefully controlled 

 conditions. In the second place, it reopened discussion of 

 what we mean when we say we have explained something. 



