XI PHENOMENA OF ORGANIC NATURE 451 



put in practice the principles that I then laid 

 down. 



I stated to you in substance, if not in words, that 

 wherever there are complex masses of phenomena 

 to be inquired into, whether they be phenomena 

 of the affairs of daily life, or whether they belong 

 to the more abstruse and difficult problems laid 

 before the philosopher, our course of proceeding 

 in unravelling that complex chain of phenomena 

 with a view to get at its cause, is always the same ; 

 in all cases we must invent an hypothesis; we 

 must place before ourselves some more or less 

 likely supposition respecting that cause ; and then, 

 having assumed an hypothesis, having supposed a 

 cause for the phenomena in question, we must 

 endeavour, on the one hand, to demonstrate our 

 hypothesis, or, on the other, to upset and reject it 

 altogether, by testing it in three ways. We must, 

 in the first place, be prepared to prove that the 

 supposed causes of the phenomena exist in nature ; 

 that they are what the logicians call vera causce 

 true causes ; in the next place, we should be pre- 

 pared to show that the assumed causes of the 

 phenomena are competent to produce such pheno- 

 mena as those which we wish to explain by them ; 

 and in the last place, we ought to be able to show 

 that no other known causes are competent to pro- 

 duce these phenomena. If we can succeed in satis- 

 fying these three conditions we shall have demon- 

 strated our hypothesis ; or rather I ought to say 



