swans might argue, by our principle, that on the
data it was _probable_ that all swans were white, and this might be a
perfectly sound argument. The argument is not disproved ny the fact that
some swans are black, because a thing may very well happen in spite of
the fact that some data render it improbable. In the case of the swans,
a man might know that colour is a very variable characteristic in many
species of animals, and that, therefore, an induction as to colour is
peculiarly liable to error. But this knowledge would be a fresh datum,
by no means proving that the probability relatively to our previous data
had been wrongly estimated. The fact, therefore, that things often fail
to fulfil our expectations is no evidence that our expectations will not
_probably_ be fulfilled in a given case or a given class of cases. Thus
our inductive principle is at any rate not capable of being _disproved_
by an appeal to experience.

The inductive principle, however, is equally incapable of being _proved_
by an appeal to experience. Experience might conceivably confirm
the inductive principle as regards the cases that have been already
examined; but as regards unexamined cases, it is the inductive principle
alone that can justify any inference from what has been examined to what
has not been examined. All arguments which, on the basis of experience,
argue as to the future or the unexperienced parts of the past or
present, assume the inductive principle; hence we can never use
experience to prove the inductive principle without begging the
question. Thus we must either accept the inductive principle on the
ground of its intrinsic evidence, or forgo all justification of our
expectations about the future. If the principle is unsound, we have no
reason to expect the sun to rise to-morrow, to expect bread to be more
nourishing than a stone, or to expect that if we throw ourselves off
the roof we shall fall. When we see what looks like our best friend
approaching us, we shall have no reason to suppose that his body is not
inhabited by the mind of our worst enemy or of some total stranger. All
our conduct is based upon associations which have worked in the past,
and which we therefore regard as likely to work in the future; and this
likelihood is dependent for its validity upon the inductive principle.

The general principles of science, such as the belief in the reign
of law, and the belief that every event must have a cause, are as