A category mistake, or category error, is a semantic or ontological error in which "things of one kind are presented as if they belonged to another", or, alternatively, a property is ascribed to a thing that could not possibly have that property. Thus the claim that "Most Americans are atheists" (untrue in 2009) is not a category mistake, since most Americans could be (contingently) atheists. On the other hand, "Most bananas are atheists" is a category mistake. This is because bananas belong to a category of things that cannot be said to have beliefs. Thomas Szasz argued that minds are not the sort of things that can be said to be diseased or ill because they belong to the wrong category and that "illness" is a term that can only be ascribed to things like the body; saying that the mind is ill is a misuse of words. Another example is the metaphor "time crawled", which if taken literally is not just false but a category mistake. To show that a category mistake has been committed one must typically show that once the phenomenon in question is properly understood, it becomes clear that the claim being made about it could not possibly be true.
The term "category-mistake" was introduced by Gilbert Ryle in his book The Concept of Mind (1949) to remove what he argued to be a confusion over the nature of mind born from Cartesian metaphysics. Ryle alleged that it was a mistake to treat the mind as an object made of an immaterial substance because predications of substance are not meaningful for a collection of dispositions and capacities.
The phrase is introduced in the first chapter. The first example is of a visitor to Oxford. The visitor, upon viewing the colleges and library, reportedly inquired “But where is the University?" The visitor's mistake is presuming that a University is part of the category "units of physical infrastructure" or some such thing, rather than the category "institutions", say, which are far more abstract and complex conglomerations of buildings, people, procedures, and so on. Ryle's second example is of a child witnessing the march-past of a division of soldiers. After having had battalions, batteries, squadrons, etc. pointed out, the child asks when is the division going to appear. 'The march-past was not a parade of battalions, batteries, squadrons and a division; it was a parade of the battalions, batteries and squadrons of a division.' (Ryle's italics) His third example is of a foreigner being shown a cricket match. After being pointed out batsmen, bowlers and fielders, the foreigner asks: 'who is left to contribute the famous element of team-spirit?'
He goes on to argue that the Cartesian dualism of mind and body rests on a category-mistake. In the philosophy of the mind, Ryle's category mistake argument can be used to support eliminative materialism. By using the argument, one can attack the existence of a separate, distinct mind. The argument concludes that minds are not conscious, but a collective predicate for a set of observable behaviors and unobservable dispositions.
- Simon Blackburn 1994. The Oxford Dictionary of Philosophy, Oxford University Press, p. 58.
- "What People Do and Do Not Believe in". Harris Interactive. 2009-12-15. Retrieved 2011-05-15.
- Gilbert Ryle The Concept of Mind, 1949. p. 16.
- MacFadden, T.G.: "Understanding the Internet: Model, Metaphor, and Analogy", Library Trends, Vol. 50, No. 1, Summer 2001, Johns Hopkins University Press, Baltimore, p.96 
- Welshon, Rex (2011). Philosophy, Neuroscience and Consciousness. Durham: Acumen. p. 28. ISBN 978-1-84465-159-7.
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