$ i95] Newton's Scientific Method 245 



195. Newton's scientific method did not differ essentially 

 from that followed by Galilei (chapter vi., 134), which 

 has been variously described as complete induction or 

 as the inverse deductive method, the difference in name 

 corresponding to a difference in the stress laid upon 

 different parts of the same gtneral process. Facts are 

 obtained by observation or experiment ; a hypothesis or 

 provisional theory is devised to account for them ; from 

 this theory are obtained, if possible by a rigorous process 

 of deductive reasoning, certain consequences capable of 

 being compared with actual facts, and the comparison is 

 then made. In some cases the first process may appear 

 as the more important, but in Newton's work the really 

 convincing part of the proof of his results lay in the 

 verification involved in the two last processes. This has 

 perhups been somewhat obscured by his famous remark, 

 Hypotheses nonfingo (I do not invent hypotheses), dissociated 

 from its context. The words occur in the conclusion of 

 the Printipia, after he has been speaking of universal 

 gravitation : 



" I have not yet been able to deduce (deducere} from 

 phenomena the reason of these properties of gravitation, and 

 I do not invent hypotheses. For any thing which cannot be 

 deduced from phenomei a should be called a hypothesis." 



Newton probably had in his mind such speculations as 

 the Cartesian vortices, which could not be deduced directly 

 from observations, and the consequences of which either 

 could not be worked out and compared with actual facts 

 or were inconsistent with them. Newton in fact rejected 

 hypotheses which were unverifiable, but he constantly made 

 hypotheses, suggested by observed facts, and verified by 

 the agreement of their consequences with fresh observed 

 facts. The extension of gravity to the moon ( 173) is a 

 good example : he was acquainted with certain facts as to 

 the motion of falling bodies and the motion of the moon ; 

 it occurred to him that the earth's attraction might extend 

 as far as the moon, and certain other facts connected with 

 Kepler's Third Law suggested the law of the inverse 

 square. If this were right, the moon's acceleration towards 

 the earth ought to have a certain value, which couM 1>- 4 



