Category mistakes are a kind of fallacy where one assigns properties or attributes to a certain thing which do not actually belong.
Consider the following common example.
Science says that the Earth orbits the Sun.
This kind of statement falsely attributes a statement to an abstract concept. Science is an abstract category to which certain methods or people belong (in the form of scientists). It is a fairly uncertain and broad concept, but it is certainly not the kind of thing that can say anything. People say things, science does not. These kinds of mistakes are not to be confused with metaphors, which often employ category mistakes for the purpose of illustrating a shared meaning.
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