In October 1932, soon after the exchange with Gödel, Carnap's thought underwent yet another major revolution. It does not appear to have occurred all at once, in a blinding flash of insight, like its predecessor in January 1931. Carnap recorded no single inspiration or breakthrough moment; we do not even know exactly when the turning point came. It seems to have been less a single insight than a continuous process that had begun at least a few months before the correspondence with Gödel, which that exchange then pushed in the direction of a new and radical resolution. The governing idea of the new attitude was the ‘principle of tolerance’. The shift toward physicalism, discussed in Chapter 9, may have helped this process along. In particular, the discussion of the empirical basis growing out of the ‘protocol sentence debate’ was the context in which the new doctrine of tolerance found its first expression.

THE PROTOCOL SENTENCE DEBATE IN 1932

Though Neurath had done much to inspire Carnap's turn to physicalism in 1930–31, he was unhappy with the paper on ‘The Physical Language’ because he saw vestiges of foundationalism in it. Also he felt himself unfairly anticipated as well as misrepresented. After an emotional correspondence with Carnap in early 1932, he sent a draft reply in May. Much of it, Carnap said, was unintelligible to him, but – as the co-editor of Erkenntnis (and as a friend) – he set to work on drastic revisions.