 There are have been four versions of the epistemological approach, each of which attempts to define how an cognitively ideal agent arrives at warranted conclusions, given an initial input. The first two of these, John L. Pollock’s theory of defeasible reasoning and the theory of semantic inheritance networks, are explicitly computational in nature. They take as input a complex, structured state, representing the data available to the agent, and they define a procedure by which new conclusions can be warranted. The third approach, based on the theory of belief change (the AGM model) developed by Alchourrón, Gärdenfors, and Makinson (Alchourrón, Gärdenfors, and Makinson 1982), instead lays down a set of conditions that an ideal process of belief change ought to satisfy. The AGM model can be used to define a nonmonotonic consequence relation that is temporary and local. This can represent reasoning that is hypothetically or counterfactually defeasible, in the sense that what “follows” from a conjunctive proposition \((p \amp q)\) need not be a superset of what “follows” from \(p\) alone. The fourth approach is that of formal argumentation theory, in which defeat is treated as a relation between arguments within a dialogue