en. ix. j PHILOSOPHY AS AN ORGANON. 243 



merit ; while that of agreement is more especially the resource 

 employed where experimentation is impossible." 



Now in astronomy we can employ only simple observation. 

 The magnitude and the inaccessibility of the phenomena 

 render it impossible for us to vary the circumstances, so that 

 experiment is out of the question. Nevertheless, here the 

 phenomena are so simple that the method of agreement alone 

 carries us far toward certainty ; and accordingly in astronomy 

 the art of observation has been brought to such a pitch of 

 perfection, and the conditions of an accurate observation are 

 so thoroughly understood, that it is here that the use of this 

 implement of induction must be studied. 



In physics, both molar and molecular, and in chemistry, 

 the phenomena become far more complicated. Yet here we 

 become able to vary the phenomena almost indefinitely ; and 

 accordingly physics and chemistry are the inductive sciences 

 par excellence, in which experiment, the great engine of in- 

 duction, is employed most successfully, and in which, there- 

 fore, is especially to be studied the proper use of the method 

 of difference. 



When we come to biology, we are met by a still greater 

 amplication of phenomena ; but according to the luminous 

 principle, first suggested by Comte, that in general our means 

 of investigation increase with the complexity of the pheno- 

 mena, we have here an additional weapon of investigation. 

 We still retain the ability to experiment ; although such is 

 the intricacy of the circumstances, and such the subtlety of 

 the causes in operation, that we can seldom apply the potent 

 method of difference. We can seldom be sure that the two 

 instances compared agree in everything save in the presence 

 or absence of the circumstance we are studying. 1 In expe- 

 rimenting upon live animals, we are liable to cause a patho- 



1 A striking illustration of this truth is furnished by the controversy now 

 going on concerning archebiosis or "spontaneous generation." See below, 

 part ii. chap. viii. 



B 2 



