 Why did Carnap decide to change directions so radically, at the age of 52? This is unusual in a philosopher, especially as in Carnap’s case it involved reading his way into a whole new field of inquiry, as well as learning a whole new branch of mathematics. We know that the idea of a “degree of confirmation” had interested him since at least the time of “Testability and Meaning”. At this time, as we saw in  section 8.1  above, he thought it could be defined only comparatively, since he rejected Reichenbach’s conflation of truth-value with frequency as well as his identification of relative frequency and confirmation (Carnap 1935b, 1936–37). And it had been the accepted wisdom in the Vienna Circle that the classical (epistemic) interpretation of probability had been entirely superseded by the frequency interpretation, partly because the latter relied on Laplace’s much-criticized principle of indifference, which they classified as metaphysical. Even Keynes (1921) had given insufficiently clear arguments and his axioms were defective, they thought, but there was interest in Wittgenstein’s logical (purely analytic) conception of probability, suggested in the Tractatus, and developed in more detail by Waismann (1930). 