popper-logic-scientific-discovery

448
new appendices
natural laws seems to me utterly unacceptable. Yet his criticism seems
to me most valuable.
(6) I am now going to explain, with the help of an example, what I
believe to be essentially Kneale’s criticism of the view that a characterization
of laws of nature as universal statements is logically sufficient and
also intuitively adequate.
Consider some extinct animal, say the moa, a huge bird whose
bones abound in some New Zealand swamps. (I have there dug for
them myself.) We decide to use the name ‘moa’ as a universal name
(rather than as a proper name; cf. section 14) of a certain biological
structure; but we ought to admit that it is of course quite possible—
and even quite credible—that no moas have ever existed in the universe,
or will ever exist, apart from those which once lived in New
Zealand; and we will assume that this credible view is correct.
Now let us assume that the biological structure of the moa organism
is of such a kind that under very favourable conditions, a moa might
easily live sixty years or longer. Let us further assume that the conditions
met by the moa in New Zealand were far from ideal (owing,
perhaps, to the presence of some virus), and that no moa ever reached
the age of fifty. In this case, the strictly universal statement ‘All moas
die before reaching the age of fifty’ will be true; for according to our
assumption, there never is, was, or will be a moa in the universe more
than fifty years of age. At the same time, this universal statement will
not be a law of nature; for according to our assumptions, it would be
possible for a moa to live longer, and it is only due to accidental or contingent
conditions—such as the co-presence of a certain virus—that in fact no
moa did live longer.
The example shows that there may be true, strictly universal statements
which have an accidental character rather than the character of true
universal laws of nature. Accordingly, the characterization of laws
of nature as strictly universal statements is logically insufficient and
intuitively inadequate.
(7) The example may also indicate in what sense natural laws may
be described as ‘principles of necessity’ or ‘principles of impossibility’,
as Kneale suggests. For according to our assumptions—assumptions
which are perfectly reasonable—it would be possible, under favourable
conditions, for a moa to reach a greater age than any moa has actually