In the last decade of the Twentieth Century architectural discourse offered a large number of manifestos which postulate that sustainable design strategies are morally or ethically sound. Drawing on a close analysis of published works that deploy an ethical or moral argument for sustainable architecture, the present paper investigates the way in which such propositions are structured. The paper is not concerned with the scientific validity of the ethical architecture argument but with its form (logic, sequence, presuppositions, predicates and suppositions). The structure of such an argument is reliant on the way in which an ethical proposition is framed in terms of three key philosophical schools of thought. These three schools of thought are explained in the paper in a way which is consistent with their conventional application in architecture. The aim of such an analysis of argument structure is primarily epistemological. Knowledge is developed through exposition, postulation and experimentation; a detailed investigation of the way in which mechanisms propagate knowledge is valuable for a critical understanding of any discipline. Regardless of whether or not sustainable architecture possesses positive moral characteristics the growth and acceptance of such arguments is, in itself, significant and worthy of detailed study.