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.
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