‘explanation’ and ‘understanding’. According to the metalinguistic version, all objects of scientific knowledge could in principle be comprehended by the same ‘universal’ language. Physicalism asserts that this is the language that speaks of physical objects. While everybody in the Circle endorsed physicalism in this sense, the understanding of its importance varied, as became clear in the socalled protocol sentence debate. (The nomological version of the unity thesis was only later clearly distinguished: whether all scientific laws could be reduced to those of physics was another matter on which Neurath came to differ.) Ostensively, this debate concerned the question of the form, content, and epistemological status of scientific evidence statements. Schlick’s unrevisable ‘affirmations’ talked about phenomenal states in statements not themselves part of the language of science (‘The Foundation of Knowledge,’ 1934, translated in Ayer, ed., Logical Positivism). Carnap’s preference changed from unrevisable statements in a primitive methodologically solipsistic protocol language that were fallibly translatable into the physicalistic system language (1931; see Unity of Science, 1934), via arbitrary revisable statements of that system language that are taken as temporary resting points in testing (1932), to revisable statements in the scientific observation language (1935; see ‘Testability and Meaning,’ Philosophy of Science, 1936–37). These changes were partly prompted by Neurath, whose own revisable ‘protocol statements’ spoke, amongst other matters, of the relation between observers and the observed in a ‘universal slang’ that mixed expressions of the physicalistically cleansed colloquial and the high scientific languages (‘Protocol Statements,’ 1932, translated in Ayer, ed., Logical Positivism). Ultimately, these proposals answered to different projects. Since all agreed that all statements of science were hypothetical, the questions of their ‘foundation’ concerned rather the very nature of Vienna Circle philosophy. For Schlick philosophy became the activity of meaning determination (inspired by Wittgenstein); Carnap pursued it as the rational reconstruction of knowledge claims concerned only with what Reichenbach called the ‘context of justification’ (its logical aspects, not the ‘context of discovery’); and Neurath replaced philosophy altogether with a naturalistic, interdisciplinary, empirical inquiry into science as a distinctive discursive practice, precluding the orthodox conception of the unity of science. The Vienna Circle was neither a monolithic nor a necessarily reductionist philosophical movement, and quick assimilation to the tradition of British empiricism mistakes its struggles with the form–content dichotomy for foundationalism, when instead sophisticated responses to the question of the presuppositions of their own theories of knowledge were being developed. In its time and place, the Circle was a minority voice; the sociopolitical dimension of its theories – stressed more by some (Neurath) than others (Schlick) – as a renewal of Enlightenment thought, ultimately against the rising tide of Blutund-Boden metaphysics, is gaining recognition. After the celebrated ‘death’ of reductionist logical positivism in the 1960s the historical Vienna Circle is reemerging as a multifaceted object of the history of analytical philosophy itself, revealing in nuce different strands of reasoning still significant for postpositivist theory of science. See also MEANING, OPERATIONALISM , PHI- LOSOPHY OF SCIENCE , REDUCTION , UNITY OF SCIENC. T.U.