 This project has its roots in early work of Mackey (1957, 1963), Ludwig (1964), and Piron (1964) aiming to characterize quantum mechanics in operational terms. This has led to the development of a framework of generalized probabilistic model. It also has connections with the investigations into quantum logic initiated by Birkhoff and von Neumann (1936) (see the entry  quantum logic and probability theory  for an overview).