Two characters are perceived as insane in the course of "The Miller's Tale." The first, Nicholas, pretends to be insane to get John's attention, while the second, John, is "holden wood" (held to be mad) by the townspeople when Nicholas and Alisoun paint him that way. The fact that neither of these characters is truly insane emphasizes the way madness may just be in the eye of the beholder. In John's case, moreover, the story links the perception of madness to the inability to make oneself heard, suggesting that the label of madness may be one to which the powerless, who have a harder time getting people to listen to them, are more vulnerable.
By portraying John's perceived madness as the result of his inability to make himself heard, "The Miller's Tale" suggests that society's powerless groups are vulnerable to a label of madness.
"The Miller's Tale" defines madness as an inability to participate in the reality of those around you.