The distinction between knowledge of specific exemplars and knowledge of their general categories is central to much theorising on the nature of semantic memory. The dissociation between exemplar and category knowledge observed in Alzheimer's Disease (AD) would appear to support this distinction, and to suggest that different neural systems are involved in the representation of exemplar and category knowledge. We review the evidence for preserved category knowledge in the semantic memory impairment of AD, and propose an alternative interpretation, according to which category and exemplar knowledge are both represented in the same distributed neural substrate. The relative preservation of category knowledge is a consequence of the greater frequency, and hence greater robustness, of the representation of attributes shared by all or most members of a category, compared to exemplar-unique attributes. We test and confirm the computational adequacy of this hypothesis in two computer simulations.