I will own I look upon my master Don Quixote as an incurable madman; although sometimes he says things, which, to my thinking, and in the opinion of all who hear them, are so reasonable and well directed, that even Satan himself could not mend them[.]

In this passage, Sancho is able to see the contradictory in the madness of Don Quixote.

Explanation:

The Duchess begged Sancho to talk to her about the deceit or madness of Don Quixote, and he explains that because he has spent a lot of time next to Don Quixote, he is able to understand his madness and also tells him that in sometimes Don Quijote seems lucid and rational. It is obvious to observe the contradictory of this that Sancho says, the madness and deception of Don Quixote is only applied to the cavalry and can even be completely reasonable