 The heart of Frege’s logicist achievement was deferred to the Grundgesetze der Arithmetik, the first volume of which appeared in 1893. This almost decade-long delay after the Grundlagen he explained in his foreword as being occasioned by some re-thinking of his Begriffsschrift (Frege 1879)—the most important innovation being the introduction of the notion of, and notation for, the Werthverlauf (value-range, or extension) of a concept. Frege had also, by the time of publication of the Grundgesetze, formulated his distinction between sense and reference, and decided to treat truth-values as objects, and indeed as referents of sentences. 