In philosophy, a category mistake, also called a category error, is a philosophical concept used to describe a statement in which the speaker presents a concept from one category as if it belongs in another, or ascribes properties from one category to concepts from another. British philosopher Gilbert Ryle (1900-1976) coined the term in his book "The Concept of Mind" (1949). The term caught on and is now widely used in philosophical discussion.
The most famous example of a category mistake is probably an example from Ryle's work. Ryle asks the reader to imagine a visitor to the city of Oxford. A guide shows the visitor all around the city, pointing out the colleges, department buildings and libraries as they go. At the end of the tour, the visitor asks "but where is the university?" For Ryle, this is a category mistake. The visitor is treating the university as if it were part of the category of buildings, rather than as what it actually is: a collection of institutions, some of which are housed in buildings of their own.
This example formed part of Ryle's criticism of the philosophy of Rene Descartes (1596-1650). To Ryle, Descartes was making a category mistake similar to that of the visitor who looked for the university while ignoring its components. Ryle argued that the mind was not separate from the body or the actions of the individual.
Some participants in philosophical discussions occasionally misapply the term incorrectly using it to mean any major error. To truly be a category mistake, a statement must attribute to something qualities that it could not possibly possess. Consider the statement "most Americans are vegetarian." This statement is not true, but it is not a category error, because theoretically it could be true: vegetarianism is a quality appropriate to the category of Americans. The statement "most Americans are about three quarters of an hour long" is this type of mistake, since it applies a quality — a duration of time — which does not apply to humans generally.
Obvious category mistakes such as this example are extremely rare, because they seem obviously false to the listener. Not all category errors are so obvious, however. In speech, a mixed metaphor, also known as catachresis, is essentially a form of category error which may be either unintentional or deliberate. In Shakespeare's Timon of Athens, for example, Philotus says "I fear 'tis deepest winter in Lord Timon's purse." In essence, this is a category mistake — purses do not have seasons — but the implied comparison is meaningful, providing a poetic comparison that illuminates and pleases at the same time.