 John Pollock’s approach to defeasible reasoning consists of enumerating a set of rules that are constructive and effectively computable, and that aim at describing how an ideal cognitive agent builds up a rich set of beliefs, beginning with a relatively sparse data set (consisting of beliefs about immediate sensory appearances, apparent memories, and such things). The inferences involved are not, for the most part, deductive. Instead, Pollock defines, first, what it is for one belief to be a prima facie reason for believing another proposition. In addition, Pollock defines what it is for one belief, say in \(p\), to be a defeater for \(q\) as a prima facie reason for \(r\). In fact. Pollock distinguishes two kinds of defeaters:  rebutting defeaters,  which are themselves prima facie reasons for believing the negation of the conclusion, and undercutting defeaters, which provide a reason for doubting that \(q\) provides any support, in the actual circumstances, for \(r\). (Pollock 1987, 484) A belief is ultimately warranted in relation to a data set (or epistemic basis) just in case it is supported by some ultimately undefeated argument proceeding from that epistemic basis.