THE PRESIDENTS ADDRESS 27 



1. First, no grreater mistake could be made than to sup- 

 pose that GaHleo was the first man to differ with Aristotle; 

 the academy of Cosena. having opposition to the peripatetic 

 philosophy as its avowed purpose, was established at Naples 

 about the time when Galileo was born : but he was the first 

 man to offer experimental evidence against the conclusions 

 of Aristotle : and in so doing he established what we now call 

 the experimental method. He was not handing on an opinion 

 which some ""dusty minded professor" had inherited from an 

 ancestor of the same type. 



Only two methods of investigation were known to the 

 ancients, the philosophical and the mathematical : to these 

 Galileo added a third, the experimental. The philosophical 

 method consisted in assuming certain general principals and 

 trying to find in them an a priori explanation of the universe. 

 Briefly described, the attempt was to stare nature out of coun- 

 tenance. Failure was inevitable, not for want of intellectual 

 acumen, but because, as every one in this assembly knows, it 

 sometimes requires a lifetime of eft'ort to explain a single de- 

 tail. \\'itness almost any chapter in Darwin's "Origin of 

 Species." Details must be mastered before one can pass to 

 general principles. 



The mathematical method consisted only in applying geo- 

 metry to certain well known areas, volumes and angles, espe- 

 cially to those angles observed in the sky. but always with 

 the idea of describing the known rather than discovering the 

 unknown: the mathematicians do not appear to have ptit any 

 deliberate questions to nature ; or as Rowland said : 



A mathematical investigation always obeys the 

 law of the conservation of knowledge : we never get 

 out more from it than we put in. The knowledge 

 may be changed in form, it may be clearer and more 

 exactly stated : but the total amount of the knowledge 

 of nature given out by the investigation is the same 

 as we started with. 



The experimental method, established mainly by Galileo, 

 not only combines the observations of the philosophers with 

 the measurements of the mathematicians, but adds deliberate 

 experiment with a distinct purpose to interrogate nature con- 

 cerning some detail of her behavior. Generalizations based 

 upon these details the experimenter reserves for a later date. 

 The high regard in which Galileo held experimental facts is 

 reflected in the following from a letter (7) to the Grand Duch- 

 ess Christina, dated 1615. He says: 



1 would entreat these wise and prudent fathers 

 to consired diligently the difference between opinion- 

 ative and demonstrative doctrines, to the end that 



