This work considers the fundamental structure of the conditions which constitute the principle of double effect. The purpose is to examine what positions need to be defended in order for the principle to be workable. The first section of the work examines the principle in its Thomistic origins. It is argued that the principle as we now know it was implicitly present in St. Thomas Aquinas\u27 treatment of self-defense found in Summa Theologiae, II-II 64, 7. The second section examines a contemporary challenge (Proportionalism) to traditional understandings of the principle. In this section it is argued that while proportionalists and advocates of traditional double effect use similar language and provide, in some cases, a similar analysis, t...