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 cause— 
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 
GGQG2 
