The fact of modern mathematics: Geometry, logic, and concept formation in Kant and Cassirer

It is now commonly accepted that any adequate history of late nineteenth and early twentieth century philosophy---and thus of the origins of analytic philosophy---must take seriously the role of Neo-Kantianism and Kant interpretation in the period. This dissertation is a contribution to our understanding of this interesting but poorly understood stage in the history of philosophy.;Kant's theory of the concepts, postulates, and proofs of geometry was informed by philosophical reflection on diagram-based geometry in the Greek synthetic tradition. However, even before the widespread acceptance of non-Euclidean geometry, the projective revolution in nineteenth century geometry eliminated diagrams from proofs and introduced "ideal" elements that could not be given a straightforward interpretation in empirical space. A Kantian like the very early Russell felt forced to regard the ideal elements as convenient fictions. The Marburg Neo-Kantians---the philosophical school that included Ernst Cassirer (1874-1945)---thought that philosophy, as "transcendental logic," needed to take the results of established pure mathematics as a "fact," not a fiction. Cassirer therefore updates Kant by rejecting the "Transcendental Aesthetic" and by using elements in Richard Dedekind's foundations of arithmetic to rework Kant's idea that the geometrical method is the "construction of concepts." He further argues that geometry is "synthetic" because it progresses when mathematicians introduce new structures (like the complex projective plane) that are not contained in the old structures, but unify them under a new point-of-view.;This new "Kantian" theory of modern mathematics, Cassirer argues, is inconsistent with the traditional theory of concept formation by abstraction. Drawing on earlier Neo-Kantian interpretations, Cassirer argues that Kant's theory of concepts as rules undermines the traditional theory of concept formation, and he gives a "transcendental" defense of the new logic of Frege and Russell. (In an appendix, I discuss the contemporaneous accounts of concept formation in Gottlob Frege and Hermann Lotze.)...