Don Quixote Volume 1, Chapter 25
After this, Sancho says he wants to go home where he can talk as much as he likes. Don Quijote gives him permission for Sancho to do so, while they remain in the mountains. Sancho asks for a logical explanation for the inexplicable, namely Don Quijote's passionate defense of a queen to a madman. He explains that this is the duty of every knight errant. Sancho asks if it is also a duty to wander around lost in the mountains looking for lunatics? Don Quijote announces that he has additional plans he is going to accomplish in these mountains. Probably fueled by witnessing Cardenio's madness, he now plans to imitate several forms of madness shown by characters in his beloved books scorned and rejected by their ladies. He tells Sancho that he must travel to Dulcinea and tell her of Don Quijote's madness over her and then bring back her reply. Sancho points out that these men were driven to it; whereas, Dulcinea has neither rejected nor been unfaithful to Don Quijote:
"'That's exactly it,' replied Don Quijote, 'that's just how beautifully I've worked it all out -- because for a knight errant to go crazy for good reason, how much is that worth? My idea is to become a lunatic for no good reason at all.'" Volume 1, Chapter 25, pg. 151
Don Quijote's pretense at madness and further references to Mambrino's basin, is starting to convince Sancho that his master is indeed batty and he tells him so. They enter a beautiful meadow, whereupon Don Quijote practices the part of a lunatic -- loudly telling the gods, nymphs and dryads of the meadow of his scorned love for Dulcinea.
As he is about to free Rocinante, Sancho asks to use the horse to travel as his donkey has been stolen. (This is the first time the reader hears of Sancho's donkey having been stolen.) Rather than waiting and witnessing Don Quijote's performance as a madman, Sancho suggests that he has seen enough examples already to provide Dulcinea with a good accounting of all the crazy, stupid things Don Quijote will be doing. As they discuss the letter to be written to Dulcinea and the letter to Don Quijote's niece (instructing her to give donkeys to Sancho), the mysterious identity of Dulcinea is revealed. Her real name is Aldonza Lorenzo and Sancho knows her and thinks quite well of her and can see why Don Quijote is smitten with her. He describes her as strong, brave, well built, funny and down-to-earth. Her talents include the ability to yell loud and well (seen in her success in calling lads home from the field -- nearly a mile away!), and making faces. He asks Don Quijote (that since she isn't a princess), what would she want with the presents Don Quijote has been sending her -- like the conquered Basque?
"[F]or what I want of Dulcinea del Toboso, she's every bit as good as the noblest princess on earth." Volume 1, Chapter 25, pg. 156
Don Quijote admits that she is part invention like the ladies that poets praise, who do not actually exist; but, invent so they have something to write about. And spoken as if it were the most rational thing to do, he adds:
"And so, to sum it all up, I perceive everything I say as absolutely true, and deficient in nothing whatever, and paint it all in my mind exactly as I want it to be." Volume 1, Chapter 25, pg. 157
The letters are written. Amid tears, Sancho gets ready to leave, but decides to see a few of Don Quijote's planned crazy antics just so he can feel honest when speaking to Dulcinea. So quick as a wink, Don Quijote takes off his pants (leaving himself naked save for his shirt) and does a couple of somersaults. Having seen more than he ever wanted to see; Sancho leaves now, confident that he will have a clear conscience when he says that his master is nuts.