 Before we proceed to the technical development, it will be helpful to introduce an important early critique of nonmonotonic logic due to the philosopher John Pollock. Pollock (1987) identifies two sources of nonmonotonicity in defeasible reasoning. An agent may believe \(\beta\) because she believes \(\alpha\) and takes \(\alpha\) to be a defeasible  reason for \(\beta\). Pollock distinguishes two kinds of  defeaters for this inference: a  rebutting defeater is a defeasible reason to believe \(\neg \beta,\) whereas an  undercutting  defeater is a reason to believe \(\neg \alpha\). Either kind of defeater may induce an agent to retract her belief in \(\beta\). Pollock's point is that since nonmonotonic logics typically do not represent the structure of an agent's reasons, they often fail to elegantly handle cases of undercutting defeat. We shall soon see several examples. 