In his prominent work, The Concept of Mind (1949), the philosopher Gilbert Ryle describes the fundamental mistake made by Descartes' dualism and much of western philosophy of the 17th and 18th centuries. The error lies in the category mistake of assigning mind and the brain as coexisting. While there is a mind and a body, the two have distinct existences and the dualist view cannot explain how one affects the other. This concept is better illustrated by one of Ryle's examples in the text. When a prospective student visits a university, s/he will see the library, the labs, the sports arena, but then many very well ask the tour guide, "but where is the university?", having been under the assumption that it's a different place altogether. The same is done by Descartes, according to Ryle, and it becomes necessary to alter the way in which we describe the entities of mind and brain.