popper-logic-scientific-discovery

Thus natural necessity or impossibility is like musical necessity or
impossibility. It is like the impossibility of a four-beat rhythm in a
classical minuet, or the impossibility of ending it on a diminished
seventh or some other dissonance. It imposes structural principles upon
the world. But it still leaves a great deal of freedom to the more
contingent singular facts—the initial conditions.
If we compare the situation in music with that of our example of the
moa, we can say: there is no musical law prohibiting the writing of a
minuet in G flat minor, but it is nevertheless quite possible that no
minuet has ever been, or will ever be, written in this unusual key. Thus
we can say that musically necessary laws may be distinguished from true
universal statements about the historical facts of musical composition.
(10) The opposite view—the view that natural laws are in no sense
contingent—seems to be the one which Kneale is advancing, if I
understand him well. To me it seems quite as mistaken as the view
which he justly criticizes—the view that laws of nature are nothing but
true universal statements.
Kneale’s view that laws of nature are necessary in the same sense in
which logical tautologies are necessary may perhaps be expressed in
religious terms thus: God may have faced the choice between creating a
physical world and not creating a physical world, but once this choice
was made, He was no longer free to choose the form, or the structure
of the world; for since this structure—the regularities of nature,
described by the laws of nature—is necessarily what it is, all He could
freely choose were the initial conditions.
It seems to me that Descartes held a view very similar to this. According
to him, all the laws of nature follow with necessity from the one
analytic principle (the essential definition of ‘body’) according to
which ‘to be a body’ means the same as ‘to be extended’; which is
taken to imply that two different bodies cannot take up the same extension,
or space. (Indeed, this principle is similar to Kneale’s standard
example—‘that nothing which is red is also green’. 11 ) But it is by
going beyond these ‘truisms’ (as Kneale calls them, stressing their
similarity to logical tautologies 12 ) that, beginning with Newton,
11 Cf. Kneale, op. cit., p. 32; see also, for example, p. 80.
12 Op. cit., p. 33.
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