popper-logic-scientific-discovery

corroboration, or how a theory stands up to tests 251
the principle of the uniformity of nature is here replaced by the postulate
of the invariance of natural laws, with respect to both space and time. I
think, therefore, that it would be a mistake to assert that natural regularities
do not change. (This would be a kind of statement that can
neither be argued against nor argued for.) What we should say is,
rather, that it is part of our definition of natural laws if we postulate that
they are to be invariant with respect to space and time; and also if we
postulate that they are to have no exceptions. Thus from a methodological
point of view, the possibility of falsifying a corroborated law
is by no means without significance. It helps us to find out what
we demand and expect from natural laws. And the ‘principle of the
uniformity of nature’ can again be regarded as a metaphysical interpretation
of a methodological rule—like its near relative, the ‘law
of causality’.
One attempt to replace metaphysical statements of this kind by principles
of method leads to the ‘principle of induction’, supposed to
govern the method of induction, and hence that of the verification of
theories. But this attempt fails, for the principle of induction is itself
metaphysical in character. As I have pointed out in section 1, the
assumption that the principle of induction is empirical leads to an
infinite regress. It could therefore only be introduced as a primitive
proposition (or a postulate, or an axiom). This would perhaps not
matter so much, were it not that the principle of induction would have
in any case to be treated as a non-falsifiable statement. For if this principle—
which is supposed to validate the inference of theories—were itself
falsifiable, then it would be falsified with the first falsified theory,
because this theory would then be a conclusion, derived with the help
of the principle of induction; and this principle, as a premise, will of
course be falsified by the modus tollens whenever a theory is falsified
which was derived from it.* 3 But this means that a falsifiable principle
of induction would be falsified anew with every advance made by
science. It would be necessary, therefore, to introduce a principle of
induction assumed not to be falsifiable. But this would amount to the
* 3 The premises of the derivation of the theory would (according to the inductivist view
here discussed) consist of the principle of induction and of observation statements. But
the latter are here tacitly assumed to be unshaken and reproducible, so that they cannot
be made responsible for the failure of the theory.