Carnap was one of the most significant philosophers of the twentieth century, and made important contributions to logic, philosophy of science, semantics, modal theory and probability. Viewed as an enfant terrible when he achieved fame in the Vienna Circle in the 1930s, Carnap is more accurately seen as one who held together its widely varying viewpoints as a coherent movement. In the 1930s he developed a daring pragmatic conventionalism according to which many traditional philosophical disputes are viewed as the expression of different linguistic frameworks, not genuine disagreements. This distinction between a language (framework) and what can be said within it was central to Carnap’s philosophy, reconciling the apparently a priori domains such as logic and mathematics with a thoroughgoing empiricism: basic logical and mathematical commitments partially constitute the choice of language. There is no uniquely correct choice among alternative logics or foundations for mathematics; it is a question of practical expedience, not truth. Thereafter, the logic and mathematics may be taken as true in virtue of that language. The remaining substantive questions, those not settled by the language alone, should be addressed only by empirical means. There is no other source of news. Beyond pure logic and mathematics, Carnap’s approach recognized within the sciences commitments aptly called a priori – those not tested straightforwardly by observable evidence, but, rather, presupposed in the gathering and manipulation of evidence. This a priori, too, is relativized to a framework and thus comports well with empiricism. The appropriate attitude towards alternative frameworks would be tolerance, and the appropriate mode of philosophizing the patient task of explicating and working out in detail the consequences of adopting this or that framework. While Carnap worked at this tirelessly and remained tolerant of alternative frameworks, his tolerance was not much imitated nor were his principles well understood and adopted. By the time of his death, philosophers were widely rejecting what they saw as logical empiricism, though often both their arguments and the views offered as improvements had been pioneered by Carnap and his associates. By his centenary, however, there emerged a new and fuller understanding of his ideas and of their importance for twentieth-century philosophy.
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