 The second of these two research traditions is philosophical. In a wide sense, belief change has been a subject of philosophical reflection since antiquity. In the twentieth century, philosophers have discussed the mechanisms by which scientific theories develop, and they have proposed criteria of rationality for revisions of probability assignments. Beginning in the 1970s, a more focused discussion has taken place on the requirements of rational belief change. Two milestones can be pointed out. The first was a series of studies conducted by Isaac Levi in the 1970s (Levi 1977, 1980). Levi posed many of the problems that have since then been major concerns in this field of research. He also provided much of the basic formal framework. William Harper’s (1977) work from the same period has also had a lasting influence. 