other people, of the past before
our individual memory begins, or of the future, we must know general
principles of some kind by means of which such inferences can be drawn.
It must be known to us that the existence of some one sort of thing, A,
is a sign of the existence of some other sort of thing, B, either at
the same time as A or at some earlier or later time, as, for example,
thunder is a sign of the earlier existence of lightning. If this were
not known to us, we could never extend our knowledge beyond the
sphere of our private experience; and this sphere, as we have seen, is
exceedingly limited. The question we have now to consider is whether
such an extension is possible, and if so, how it is effected.

Let us take as an illustration a matter about which none of us, in fact,
feel the slightest doubt. We are all convinced that the sun will rise
to-morrow. Why? Is this belief a mere blind outcome of past experience,
or can it be justified as a reasonable belief? It is not easy to find
a test by which to judge whether a belief of this kind is reasonable or
not, but we can at least ascertain what sort of general beliefs would
suffice, if true, to justify the judgement that the sun will rise
to-morrow, and the many other similar judgements upon which our actions
are based.

It is obvious that if we are asked why we believe that the sun will rise
to-morrow, we shall naturally answer 'Because it always has risen every
day'. We have a firm belief that it will rise in the future, because it
has risen in the past. If we are challenged as to why we believe that
it will continue to rise as heretofore, we may appeal to the laws of
motion: the earth, we shall say, is a freely rotating body, and such
bodies do not cease to rotate unless something interferes from outside,
and there is nothing outside to interfere with the earth between now and
to-morrow. Of course it might be doubted whether we are quite certain
that there is nothing outside to interfere, but this is not the
interesting doubt. The interesting doubt is as to whether the laws
of motion will remain in operation until to-morrow. If this doubt is
raised, we find ourselves in the same position as when the doubt about
the sunrise was first raised.

The _only_ reason for believing that the laws of motion will remain in
operation is that they have operated hitherto, so far as our knowledge
of the past enables us to judge. It is true that we have a greater body
of e