For medieval thinkers, a prominent philosophical, religious and legal problem concerned how to distinguish between the ‘will-not’ and the ‘can-not’. Amassing medieval evidence for the characterization these 'types', this chapter considers the tension between people regarded as not wanting to do something and people incapable of doing something despite perhaps wanting to. The 'genuine fool' was accorded preferential treatment in all these realms, but the 'pretend fool' was regarded with suspicion, and was perceived as morally dubious, even dangerous. Precisely because cognitive disability is not something writ large on the body, like a crippled limb, medieval commentators were worried by it, just as they were worried by deafness (equally invisible and also causing communication and moral issues). It is the behaviour rather than the physique that is highlighted as being different from 'the norm'. It is a sign of more modern times that physical appearance comes to be more strongly linked to cognitive disability. Medieval children appear to have been categorised by their learning ability as expressed through behaviour, not physiognomy.