Rudolf Carnap was the philosophically most articulate member of the Vienna Circle in the 1920s and 1930s, and later of the movement that came to be known in the United States as logical empiricism. During his lifetime, he was respected among analytic philosophers as the proponent of a number of ambitious language projects, especially, in his later years, a system of inductive logic. The philosophical agenda underlying these technical projects, however, remained largely implicit; only disconnected fragments of this agenda, often reduced to superficial slogans, gained some currency. Subsequent generations, quite reasonably, discarded these fragments with some contempt. The coherent and powerful view that Carnap actually held (and partly articulated), of which the ambitious technical projects were manifestations and illustrations, but not explicit statements, has only begun to be unearthed. As a result, the view of Carnap held during his lifetime and since his...