this very reason possible that the examination of it may be of the very
greatest importance. To explain what has hitherto received no
explanation constitutes the very essence of scientific progress. The
observation may be imperfect, and may at once become explicable as soon
as it is made complete; or, what is of far more value, it may be an
instance of the operation of a new law not previously known, modifying
and perhaps absorbing the law up to that time accepted. When it was
first noticed in Galileo's time that water would not ascend in the
suction pipe of a pump to a greater height than 32 feet, the old law
that nature abhors a vacuum was modified, and the reasons why and the
conditions under which Nature abhors a vacuum were discovered. The
suction of fluids was brought under the general law of mechanical
pressure. The doctrine that Nature abhorred a vacuum had been a fair
generalization and expression of the facts of this kind that up to that
time had been observed. A new fact was observed which would not fall
under the rule. The examination of this fact led to the old rule being
superseded; and Science advanced a great step at once. So in our own day
was the planet Neptune discovered by the observation of certain facts
which could not be squared with the facts previously observed unless the
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