As a linguistic philosopher, a significant portion of Ryle's argument is devoted to analyzing what he perceives as philosophical errors based in conceptual use of language. His critique of Cartesian dualism refers to it as a "category mistake". Category mistakes, such as the ones Ryle points out, are made by people who do not know how to properly wield the concepts with which they are working. Their puzzles arise from the inability to use certain items in human language. A much-cited example is of a foreign visitor being shown round Oxford (which has no campus) and after having been shown colleges, libraries, laboratories and playing fields asks in puzzlement "But where is the university?" The answer is, of course, all of these.