 Another strand of Carnap’s indifference to ontology was his lifelong focus on structures as the main objects of knowledge. Even as early as 1922, Carnap had thought that mathematics, correctly understood, is not so much the study of quantity and number as the study of structures; Kant’s dictum that something is scientific just insofar as it is mathematical was now superseded, he thought, by his new dictum that something is scientific just insofar as it is structural (Carnap 1922b: p. cl[r], translated and quoted in Carus 2007a: 163). And if the object of our knowledge is not the particular or the qualitative but the structural, then, as Hermann Weyl had said, “A science can determine its subject matter only up to an isomorphic representation”.