160 THE LIFE OF SCIENCE 



Leonardo da Vinci its first deliberate vindicator. After that, its 

 progress became more and more rapid, and, by the beginning of 

 the following century, experimental philosophy was admirably 

 explained by another Tuscan, Galileo, the herald of modern 

 science. 



Thus if we take a very broad view of the history of science, we 

 may distinguish in it four main phases. The first is the empirical 

 development of Egyptian and Mesopotamian knowledge. The 

 second is the building of a rational foundation of astounding 

 beauty and strength by the Greeks. The third, and until recently 

 the least known, is the medieval period — many centuries of 

 groping. Immense efforts were spent to solve pseudo-problems, 

 chiefly to conciliate the results of Greek philosophy with religious 

 dogmas of various kinds. Such efforts were naturally sterile, as 

 far as their main object was concerned, but they brought into 

 being many incidental results. The main result, as I have just 

 explained, was the incubation of the experimental spirit. Its final 

 emergence marks the transition between the third period and the 

 fourth, which is the period of modern science. Note that out of 

 these four periods the first is entirely oriental, the third is mostly 

 but not exclusively so; the second and fourth are exclusively 

 western. 



To return to the fourth period — which is still continuing — the 

 final establishment of the experimental philosophy was indeed its 

 main distinction, its standard, and its glory. Not only did the new 

 method open the path to untold and unimaginable discoveries, but 

 it put an end to unprofitable quests and idle discussions; it broke 

 the vicious circles wherein philosophers had been obstinately 

 turning for more than a thousand years. It was simple enough in 

 itself, but could not be understood as long as a series of intel- 

 lectual prejudices obscured man's vision. It may be summed up 

 as follows : Establish the facts by direct, frequent, and careful ob- 

 servations, and check them repeatedly one against the other; these 

 facts will be your premises. V/hen many variables are related, find 

 out what happens when only one is allowed to vary, the others re- 



