 Over the past forty years, researchers in artificial intelligence have created many different logics for defeasible inference, often developed to model a specific kind of defeasible reasoning. See the entry on  non-monotonic logic  for an excellent overview. In view of this profusion of specialized logics, non-monotonic logic investigates which properties a logic of defeasible consequence must have in order to count as a  logic at all. (See Gabbay (1985) for the origins of this abstract point of view.) Non-monotonic logic provides a crucial  lingua franca for comparing different logics of defeasible inference. It is also extremely apt for the purposes of this article because it allows us to compare different normative theories of how beliefs ought to be updated in light of new evidence, as well as theories of how full and partial beliefs ought to relate to each other. 