  In short, it is not simply that the conception of evidence gleaned from exemplary scientific practice differs from the conception of evidence that traditional epistemology seemed to demand (as would be the case if, for example, one was somewhat more inclusive than the other). Rather, what seemed to be one of the characteristic features of scientific evidence, viz. its potential publicity, is the exact contrary of one of the characteristic features of evidence as construed by much traditional epistemology, viz. its privacy. Such a tension was bound to generate a certain amount of dissonance. One historically noteworthy manifestation of this dissonance was the protracted debate within the positivist tradition over the nature of ‘protocol sentences’. Here, Carnap's odyssey is perhaps the most illuminating. Early in his philosophical career, Carnap, under the influence of Russell and Ernst Mach (and through them, the tradition of classical empiricism) took sense data as the ultimate evidence for all of our empirical knowledge.  (Indeed, his early work Der Logischer Aufbau der Welt (1928) is largely devoted to the Russellian project of ‘constructing’ the external world out of sense data.) During this period, Carnap maintained that the protocol sentences that serve as the ‘confirmation basis’ for scientific theories refer to sense data or (to be more precise) ‘the sensing of sense data’. However, under the influence of both Popper and Otto Neurath, Carnap ultimately abandoned this view of protocol sentences in favor of one on which such sentences refer not to private mental events but rather to public objects and states of affairs. As recounted in his philosophical autobiography, the primary motivation for this change of heart was the growing conviction that the conception of evidence as private mental states rendered it inadequate to ground the intersubjectivity and objectivity of  science.[37]  In this area as in others, the evolution of Carnap's own views was crucial to the evolution of the views of the Vienna Circle as a whole. The episode was well-recounted years later by Ayer: 