 Secondly, even if we can verify that the standards of mutual exclusivity and exhaustiveness are met, these conditions alone are far too weak to uniquely pick out a system of categories. Provided one accepts the law of the excluded middle, an endless supply of mutually exclusive and exhaustive classifications can be generated at will: we can divide things into the spatio-temporally located and the not-spatio-temporally-located, the intentional and the non-intentional, the extended and the non-extended, to name but a few of the more relevant ways in which things could be divided. Indeed one of the sources of puzzlement about categories comes from the fact that philosophers have selected so many different sorts of divide as the fundamental category difference – for Descartes, the extended and the thinking (unextended), for Chisholm the contingent and the necessary, for Hoffman and Rosenkrantz the concrete and the abstract, and so on. Thus another reason for skepticism about the existence of a unique set of categories comes from the fact that categories are supposed to be the most abstract genera under which things (may) fall. But from any given entity, abstraction may apparently be done in a variety of ways – even if we are careful to do so in ways that ensure mutual exclusivity and exhaustiveness.