 In spite of the range of alternatives considered, the Aufbau’s main focus on an empiricist, or phenomenological, model of knowledge in terms of the immediate experiential basis was quickly understood by fellow members of the Vienna Circle as manifesting three philosophical positions: reductionism, atomism and foundationalism. Reductionism took one set of terms to be fundamental or primitive; the rest would be logically connected to them. The objectivity, rationality and empirical content and justification of scientific knowledge depended on the resulting vertical integration. Atomism, especially in Neurath’s reading, was manifest semantically and syntactically: semantically, in the analyzability of a term; syntactically or structurally, in the elementary structure of protocols in terms of a single experiential term—‘red circle here now’—; it also appeared in the possibility of an individual testing relation of a theoretical statement to one of more experiential statements. Note that this version of atomism differs from the atomistic positivism about the form of elementary experience that Carnap attributed to Mach, in contrast with his own, scientifically updated and preferred Gestalt-doctrine of a unified field of experience at a given time. The ‘atomic’ concepts of the experiential basis are the result of a ‘quasi-analysis’.