 An argument is self-defeating if it supports a defeater for one of its own defeasible steps. Here is an interesting example: (1) Robert says that the elephant beside him looks pink. (2) Robert’s color vision becomes unreliable in the presence of pink elephants. Ordinarily, belief 1 would support the conclusion that the elephant is pink, but this conclusion undercuts the argument, thanks to belief 2. Thus, the argument that the elephant is pink is self-defeating. Pollock argues that all self-defeating arguments should be rejected, and that they should not be allowed to defeat other arguments. In addition, a set of nodes can experience mutual destruction or collective defeat if each member of the set is defeated by some other member, and no member of the set is defeated by an undefeated node that is outside the set.