 The a priori kind of knowledge claims expressed in exact theory was circumscribed to the analytic, voided of universality and necessity and deflated by Schlick and Reichenbach to the character of definitions and conventions (following Poincaré). The synthetic kind of knowledge claim was exclusively a posteriori, grounded in experience. The dominating, and later popularized, position was that revisable theoretical scientific statements should stand in appropriate logical relations to unrevisable statements about elementary observations (data), called ‘control sentences’ or ‘protocol sentences’. Such relations would provide theoretical terms with cognitive meaning or sense, and theoretical statements with verification. The form, role and status of protocol statements thus became the topic of an important philosophical debate in the early 1930s involving logical-empiricist philosophers such as Carnap, Moritz Schlick, Edgar Zilsel and Otto Neurath, and others such as Karl Popper. At stake in the debate was precisely the very articulation of a philosophy of scientific knowledge, hence its importance and also the diversity of positions that animated it.