easonable man, agrees
with you, and is convinced that you are quite right in the conclusion
you have drawn. He believes, although perhaps he does not know he
believes it, that the more extensive verifications are,--that the more
frequently experiments have been made, and results of the same kind
arrived at,--that the more varied the conditions under which the same
results are attained, the more certain is the ultimate conclusion, and
he disputes the question no further. He sees that the experiment has
been tried under all sorts of conditions, as to time, place, and people,
with the same result; and he says with you, therefore, that the law you
have laid down must be a good one, and he must believe it.

In science we do the same thing;--the philosopher exercises precisely
the same faculties, though in a much more delicate manner. In scientific
inquiry it becomes a matter of duty to expose a supposed law to every
possible kind of verification, and to take care, moreover, that this is
done intentionally, and not left to a mere accident, as in the case of
the apples. And in science, as in common life, our confidence in a law
is in exact proportion to the absence of variation in the result of our
experimental verifications. For instance, if you let go your grasp of
an article you may have in your hand, it will immediately fall to
the ground. That is a very common verification of one of the best
established laws of nature--that of gravitation. The method by which men
of science establish the existence of that law is exactly the same as
that by which we have established the trivial proposition about
the sourness of hard and green apples. But we believe it in such an
extensive, thorough, and unhesitating manner because the universal
experience of mankind verifies it, and we can verify it ourselves at any
time; and that is the strongest possible foundation on which any natural
law can rest.

So much, then, by way of proof that the method of establishing laws in
science is exactly the same as that pursued in common life. Let us now
turn to another matter (though really it is but another phase of the
same question), and that is, the method by which, from the relations of
certain phenomena, we prove that some stand in the position of causes
towards the others.

I want to put the case clearly before you, and I will therefore show you
what I mean by another familiar example. I will suppose that one of you,
on coming down in the mo