Law of Gravitation was to be corrected. The result in this case was not
the discovery of a new Law but of a new Planet; and consequently a great
confirmation of the old Law. But in each case and in every similar case
the investigation of the newly observed fact proceeds on the assumption
that Nature will be found uniform, and on no other assumption can
Science proceed at all.

Now it is this assumption which must be first examined. What is its
source? What is its justification? What, if any, are its limits?

It is not an assumption that belongs to Science only. It is in some
form or other at the bottom of all our daily life. We eat our food on
the assumption that it will nourish us to-day as it nourished us
yesterday. We deal with our neighbours in the belief that we may safely
trust those now whom we have trusted and safely trusted heretofore. We
never take a journey without assuming that wood and iron will hold a
carriage together, that wheels will roll upon axles, that steam will
expand and drive the piston of an engine, that porters and stokers and
engine-drivers will do their accustomed duties. Our crops are sown in
the belief that the earth will work its usual chemistry, that heat and
light and rain will come in their turn and have their usual effects, and
the harvest will be ready for our gathering in the autumn. Look on while
a man is tried for his life before a jury. Every tittle of the evidence
is valued both by the judge and jury according to its agreement or
disagreement with what we believe to be the laws of Nature, and if a
witness asserts that something happened which, as far as we know, never
happened at any other time since the world began, we set his evidence
aside as incredible. And the prisoner is condemned if the facts before
us, interpreted on the assumption that the ordinary laws of Nature have
held their course, appear to prove his guilt.