Dualism about undercutting defeat

Most philosophers agree that the distinction between rebutting and undercutting defeaters is sound. Recently, however, there has been much debate over the nature of and relationship between rebutting and undercutting defeaters. Among the things that have been argued about is whether undercutting defeat, in contrast to rebutting defeat, require higher-order commitment, i.e., a belief regarding the link between the source of justification and the target proposition. This paper examines the debate and argues that whether or not undercutting defeaters require higher-order commitments to this effect depends on the relevant defeatee at issue. A belief might fail to be rational in at least two ways-either by failing to correctly respond to one's reasons or by failing to be coherent with one's other attitudes. According to dualism about rationality these failings reflect two genuine and distinct kinds of rationality: substantive and structural rationality. In line with this view, the paper suggests that we should make a parallel distinction between substantive and structural undercutting defeaters. An advantage of classifying undercutting defeaters in this way is that it helps to disentangle the current debate. Another interesting result is that the proposed distinction lends further support to dualism about rationality.