 There are a variety of methods of classifying promissory theories in the literature. Atiyah (1981), Downie (1985), Vitek (1993), Shiffrin (2008), Owens (2012), van Roojen (2013), Liberto (2016b), and many others have given the matter some attention, and while there is significant overlap, there are some differences as well. Typically theorists sort theories by reference to the way in which promissory obligations are generated. On this standard taxonomy the main types of promissory theory are: Normative power views, Conventionalist views, Expectation views, and Joint Commitment views