 The ability to apply criteria in order to ensure that one’s performance is successful is like showing a ticket in order to ensure one’s right to travel by rail (1950, 239–40). It would be a category mistake to imagine that the ticket itself plays a role in the explanation of the train journey on the same level as the pistons, levers, and tracks. So, too (we might add), would it be a category mistake to imagine that reasons, for example, play a role in the explanation of action on (almost) the same level as the internal processes that explain (in a different sense) the body’s motions; or that meanings or understandings play a role in the explanation of language use on (almost) the same level as the internal processes that explain (again, in a different sense) vocalisations. But just this type of category error seems to be made by those who construe the relevant mental phenomena, including understanding, as inner causal  events.[9]