The view of Quixote as insane is something that many literary critics do accept. You will probably find an equal number who will disagree with it and view Quixote's actions in the light of a dreamer in a world that fails to validate the value of dreams. The power of Cervantes's work is that he enables both viewpoints to be vigorously debated and discussed.
In writing about a bridge between reality and fiction, Gaffney brings out the idea of Quixote's insanity. Gaffney suggests that this is something Cervantes establishes early on in the text. For Gaffney, Quixote's insanity is essential to his characterization. Lines that describe Quixote "experiencing a lack of sleep" die to an "excess of reading" as well as "impossible absurdities" are evidence that he feels helps to emphasize insanity in the text. Gaffney points to how the innkeeper and the wenches go along with Quixote's plan primarily because they think he is insane. For Gaffney, the bridge between reality and fiction lies in Quixote's insanity.
In Williamson's analysis of Quixote, insanity is the reason why he feels the work is comic in nature. Williamson's analysis stems out of what he feels as the "Romantic misreading" of Cervantes' work. This misreading does not see Quixote as insane, but rather as someone whose capacity to dream is tragic. Williamson's argument lies in how Cervantes sought to construct his work as a comedy, something that is more unifying than anything else. Its comic nature is evident in how Quixote is insane. For Williamson, to not see Quixote as insane loses Cervantes' intent, something that Williamson feels is essential in understanding its power and effectiveness as a work of literature.
Finally, I would examine Erich Auberbach's writings on the subject. Auerbach is similar to Williamson in his argument that Quixote's insanity is Cervantes's intent. Auerbach seeks to dismiss what Williamson has termed as the "Romantic" understanding of Quixote in suggesting that insanity was exactly the driving force behind his characterization. Auerbach argues that "the society surrounding Don Quixote is well structured; it is the demented knight who is out of place, and, once he recovers his sanity, he returns to the established, accepted, and unchallenged order." Examining these literary critics; views and others like them can help substantiate the idea that Quixote was constructed as an insane character.