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 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 causae'--true causes;--in the next place, we
should be prepared to show that the assumed causes of the phenomena are
competent to produce such phenomena 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 produce those phenomena. If we can
succeed in satisfying these three conditions we shall have demonstrated
our hypothesis; or rather I ought to say we shall have proved it as far
as certainty is possible for us; for, after all, there is no one of our
surest convictions which may not be upset, or at any rate modified by a
further accession of knowledge. It was because it satisfied these
conditions that we accepted the hypothesis as to the disappearance of
the tea-pot and spoons in the case I supposed in a previous lecture; we
found that our hypothesis on that subject was tenable and valid, because
the supposed cause existed in nature, because it was competent to
account for the phenomena, and because no other known cause was
competent to account for them; and it is upon similar grounds that any
hypothesis you choose to name is accepted in science as tenable and
valid.