This dissertation presents an interpretation of the writings of the German architect and theorist Gottfried Semper (1803-1879). In his theoretical works, Semper addressed two main concerns. One was to develop an understanding of the ontological significance of art and architecture, the other was to render art and architecture the object of scientific investigation and prediction. The dissertation investigates the background and implications of these conflicting concerns. Semper located the origins of architecture in ritual action, more specifically, in the way such action was embodied in the techniques and motifs of artistic making. In doing this, he broke with the neo-classical notion of the origin of architecture as a building type, a real or ideal 'primitive hut'. This reinterpretation allowed Semper to develop original reflections on architectural meaning, emphasising architecture as a creative interpretation of human actions and situations rather than as a formal-stylistic phenomenon. Situating Semper in the context of neo-classical and romantic aesthetics, the first part of the dissertation investigates the significance of this reinterpretation. I argue that Semper, by emphasising the ontological and anthropological significance of artistic making, approximated a 'poetics of architecture' in an Aristotelian sense, in which architecture is understood as mimesis of praxis. On the basis of his revised genealogy of art and architecture, Semper sought a 'logical method' by which to establish a scientific explanation of the process and products of artistic making. This 'practical aesthetics', as he called it, was to serve as a vehicle for the interpretation, diffusion and production of art and architecture. The second part of the dissertation investigates this ambition, focusing in particular on Semper's reliance on the comparative method. By examining the use of the comparative method in fields like anatomy, linguistics and sociology - all mentioned by Semper as methodological ideals - the dissertation attempts to elucidate the epistemological and methodological presuppositions of Semper's practical aesthetics. Semper's 'poetics' and his 'practical aesthetics' form an ambiguous alliance. While the poetics presented art and artistic making as an ontological and anthropological phenomenon, the practical aesthetics rendered art a matter of epistemology and method. While previous Semper scholarship has tended to treat these aspects in isolation, I will look for the intellectual framework that allowed Semper to conceive of them as a unity. The third part of the dissertation locates this framework in the philosophical outlook of historicism. By investigating the historicist fusion of romanticism and positivism, I seek to understand the implications as well as the limits of Semper's 'method of inventing.'