 Carnap is best known, though, for his application of structuralism first to epistemology, in the Aufbau, and then to mathematics itself, in the Syntax and later work, as well as to mathematically formulated scientific theories. The frameworks discussed in section 1.2 above were always structural ones, in which Carnap invariably abstracted from the actual instantiation of the structures in question in actual languages or conceptual systems, with the result that even some of his closest allies and associates misunderstood him, including (among others) Russell, Neurath, Popper, and Quine (see  sections 3  through 8 below for details). The articulation of these structural frameworks is discussed in the supplements  Aufbau,  Logical Syntax of Language,  Semantics,  and  Reconstruction of Scientific Theories.