What Carnap calls an "explication" is a procedure by which an inexact, pre-scientific concept – the "explicandum" – is replaced by a precisely defined one – the "explicatum". This meaning of the term was introduced in the context of Carnap"s work on confirmation theory, for which a concept of probability more exact than the vague ordinary one was needed (Carnap 1945b). The general problem of explicating a concept was taken up again and explained in a more precise way in Meaning and Necessity (1947, §2), in which Carnap proposed his own explication of a whole series of semantic concepts such as logical truth, logical necessity, proposition, extension, intension, and synonymy. A few years later, the general procedure of explication was recognized as having such a methodological importance that the first chapter of Logical Foundations of Probability (1950b) was entirely devoted to its detailed exposition. Later, Carnap often described his own work in terms of explication (explication of analyticity, of belief sentences, of empirical meaning, etc.) and in the preface he wrote for the second edition of the Aufbau in 1961, he even reinterpreted his earlier work on the rational reconstruction of concepts in terms of explication (Carnap 1928a/1961, p. v).