THE DEDUCTIVE METHOD. 545 



be really the causes by which the phenomena are pro- 

 duced. Thus it was very reasonably deemed an essen- 

 tial requisite of any true theory of the causes of the 

 celestial motions, that it should lead by deduction 

 to Kepler's laws : which, accordingly, the Newtonian 

 theory did. 



Tn order, therefore, to facilitate the verification of 

 theories obtained by deduction, it is important that as 

 many as possible of the empirical laws of the pheno- 

 mena should be ascertained, by a comparison of 

 instances, conformably to the Method of Agreement : 

 as well as (it must be added) that the phenomena 

 themselves should be described, in the most compre- 

 hensive as well as accurate manner possible ; by 

 collecting from the observation of parts, the simplest 

 possible correct expression for the corresponding 

 wholes : as when the series of the observed places of a 

 planet was first expressed by a system of epicycles, and 

 subsequently by an ellipse. 



It is worth remarking, that complex instances 

 which would have been of no use for the discovery of 

 the simple laws into which we ultimately analyze 

 their phenomena, nevertheless, when they have served 

 to verify the analysis, become additional evidence of 

 the laws themselves. Although we could not have 

 got at the law from complex cases, still when the law, 

 got at otherwise, is found to be in accordance with 

 the result of a complex case, that case becomes a new 

 experiment on the law, and helps to confirm what it 

 did not assist us to discover. It is a new trial of the 

 principle in a different set of circumstances ; and occa- 

 sionally serves to eliminate some circumstance not pre- 

 viously excluded,, and to effect the exclusion of which, 

 might require an experiment impossible to be executed. 

 This was strikingly conspicuous in the example for- 



VOL. I. 2 N 



