a certain
number of times causes animals and men to expect that it will happen
again. Thus our instincts certainly cause us to believe that the sun
will rise to-morrow, but we may be in no better a position than the
chicken which unexpectedly has its neck wrung. We have therefore to
distinguish the fact that past uniformities _cause_ expectations as to
the future, from the question whether there is any reasonable ground for
giving weight to such expectations after the question of their validity
has been raised.

The problem we have to discuss is whether there is any reason for
believing in what is called 'the uniformity of nature'. The belief in
the uniformity of nature is the belief that everything that has happened
or will happen is an instance of some general law to which there are no
exceptions. The crude expectations which we have been considering are
all subject to exceptions, and therefore liable to disappoint those who
entertain them. But science habitually assumes, at least as a working
hypothesis, that general rules which have exceptions can be replaced by
general rules which have no exceptions. 'Unsupported bodies in air fall'
is a general rule to which balloons and aeroplanes are exceptions. But
the laws of motion and the law of gravitation, which account for the
fact that most bodies fall, also account for the fact that balloons and
aeroplanes can rise; thus the laws of motion and the law of gravitation
are not subject to these exceptions.

The belief that the sun will rise to-morrow might be falsified if the
earth came suddenly into contact with a large body which destroyed its
rotation; but the laws of motion and the law of gravitation would not
be infringed by such an event. The business of science is to find
uniformities, such as the laws of motion and the law of gravitation,
to which, so far as our experience extends, there are no exceptions.
In this search science has been remarkably successful, and it may be
conceded that such uniformities have held hitherto. This brings us back
to the question: Have we any reason, assuming that they have always held
in the past, to suppose that they will hold in the future?

It has been argued that we have reason to know that the future will
resemble the past, because what was the future has constantly become the
past, and has always been found to resemble the past, so that we really
have experience of the future, namely of times which were formerly
future, whic