In the early stages of their intellectual development, the founding fathers of logical positivism were at least as close to Kant as they were to classical positivism. We have just seen that Schlick tried to argue for the essential unity of the epistemological approaches of Kant and Avenarius, and we have also seen the strong Kantian inspiration of his Allgemeine Erkenntnislehre. Around 1920 Reichenbach judged Kantianism to be the most appropriate standpoint for interpreting the theory of relativity. At about the same time, Carnap was writing a Kantian-style meditation on the nature of space, and he would devote most of the remainder of the decade to that most Kantian of projects, the development of a theory of constitution (Chapter 11). There can be little doubt that logical positivism started as a branch of neo-Kantianism differing from its rivals in that movement only in its concern for clarity and its appreciation of science as a model for epistemology. Yet these differences would, in the end, make all the difference.

The 1920s were a soul-searching decade for the logical positivists. During those years they struggled with the Kantian language in which they had chosen to couch their views of science and knowledge. Very slowly they came to realize how inadequate that language was to the message they were trying to convey.