Take a look at Don Quixote as a whole, and you'll see that our protagonist spends less than 0.5% of the thing in his right mind.
From the beginning, we hear that he is a country gentleman with enough money to never work again. But his idleness allows him to read a lot of adventure books. Then with some good ol' fashioned 17th-century science, the narrator tells us that "the moisture of [Don Quixote's] brain was exhausted" from so much reading that he went crazy (220.127.116.11).
So let that be a lesson to all of you: be sure to keep your brains nice and moist… by not reading… Okay, maybe the science of Cervantes's time wasn't that great.
In any case, it isn't long before Don Quixote decides to slap on an old suit of armor and ride around on his horse pretending he's a knight-errant, or basically the 17th century's version of a superhero. The interesting thing about Don Quixote's "madness," though, is that this dude is really good at backing up his weird fantasies with solid arguments.
In several places in the book, in fact, people try to tell Don Quixote that the knights he reads about in books never existed. But he has an incredible knack for saying ten totally sane things in a row, then throwing in one major crazy thing. It's tough to argue with him because he keeps tangling up what's false and true until true and false and hard to untangle.
It's not as if Don Quixote is totally, 100% mental. He's actually a pretty smart guy. He's just got one screw loose out of ten, and that's enough to send him on some wild and crazy—but actually kind of logical—adventures. There's a method to his madness.
In one instance, Don Quixote argues with a religious canon who finds himself "amazed at Don Quixote's methodical and orderly madness" (18.104.22.168). The key words here are "methodical" and "orderly." In many cases of madness, you'll find that people's brains are incredibly good at explaining away anything that doesn't fit with the way they want to view the world. In these cases, the brain tends to throw out information it doesn't like and hold on tightly to information that it does.
We're not quite sure why, but Don Quixote forgets completely about his mad ideas once he's on his deathbed. He even claims, "Now, I perceive [the] nonsense and impertinence" of his knight adventure books (22.214.171.124). It is important to know that books about knights-errant were extremely popular when Cervantes was writing Don Quixote. Luckily, we modern folks are too serious to ever get too caught up in something fictional…
At many points in this book, Cervantes makes a point of reminding us that for all his madness, Don Quixote is a highly educated and wise man. Late in the book, for example, Sancho is blown away by the Don's knowledge of literature and holy saints, wondering "how [Don Quixote] should come to know all these things; and fancied there was not that history or adventure in the world, but he had it at his fingers' end" (126.96.36.199).
Further, Don Quixote has more than just book learnin' going for him. He is also—once you cut through the crazy—a wise and noble man. When the Duke from Part 2 of the book gets a copy of Don Quixote's written advice to Sancho, for example, he and the Duchess find "a fresh occasion of admiring the mixture of Don Quixote's good sense and extravagance" (188.8.131.52). In other words, people tend to deeply admire and respect 95% of everything Don Quixote says and does. But then they remember that he's wearing an old suit of armor and trying to be a superhero, and think to themselves, "Oh yeah, he's still crazy."
Making Don Quixote a very sensible man with one weird hang-up helps Cervantes make him a more interesting character. The mixture of intelligence and foolishness makes it a bit more believable that a bored old man like Quixote would actually sit up in his library one day and decide to become a hero.
Further, Don Quixote's intelligence helps Cervantes make a worthwhile point about not letting people's poor qualities completely overshadow their good ones. It's important to acknowledge both the bad and the good when judging someone.
Now, it can be easy to write off Don Quixote's adventures as the crazy whims of a deluded old geezer. Don Quixote himself seems to believe this when his brain cells recover at the end of the novel. But it's not quite as easy as that. Maybe he didn't actually fight dragons and monsters and save princesses in distress, but he did do a lot of good for some people, even if that good was just making them laugh and enjoy life a little more.
So it's a legitimate question to ask: is Don Quixote actually a successful knight-errant, in any way? What is the difference between fact and fiction? How are they related? Are they completely separate things? Do people enjoy being around Don Quixote so much because he actually brings something unusual into their lives, something they actually want? The Duke and the Duchess certainly seem to enjoy his company for this reason.
Now, we're not saying that windmills could actually ever be monsters, but maybe ol' Don Q. has a point. Imagination is part of reality, after all. We don't have to let ours run as wild as Don Quixote's does, but what's the point of doing without it completely?
Now as we all know, you can't go riding around like a glorious knight without professing your love to a beautiful woman in a far-off castle. Love is pretty serious business in medieval romances (if you don't believe us, take a look at this one): it's all about pining, suffering, and fighting for an idealized, impossible love that (maybe) turns you into a better, more ideal person.
So that gets Don Quixote to thinking…
He remembers that "[n]ear the place where he lived dwelt a good likely country lass, for whom he had formerly had a sort of inclination" (184.108.40.206). The girl's name is Aldonza Lorenzo, but the truth is that Don Quixote has never actually seen her; he's just heard some good things about her. So, what the heck, he decides to rename her Dulcinea del Toboso to make her sound more like a princess, and for the rest of the book, he claims to do everything he does out of his love for her.
Now this would all be well and good if it didn't lead Don Quixote to actually start attacking people over it. The first time he meets a large group of riders on the road, Don Quixote demands that they "acknowledge and confess, that there is not in the universe a more beautiful damsel than the Empress de la Mancha, the peerless Dulcinea del Toboso" (220.127.116.11). People have no clue who it is he's talking about, and when they say so, he attacks them for their sauciness.
Even though Don Quixote chooses Dulcinea out of convenience in order to flatter his fantasies, the guy is crazy loyal to her. In several parts of this novel, Don Quixote is convinced that beautiful young women are throwing themselves at him. But he resists his sexual urges and endlessly pledges his loyalty to Dulcinea. His biggest test comes toward the end of the book, when the young Altisidora (in a mocking way) keeps writing songs for him and professing her love. But the Don answers over and over that "Dulcinea alone can soften my manly temper, and mould me as she pleases" (18.104.22.168). You have to hand it to the guy. He may be nuts, but he sticks to his principles.
So, okay, it's totally ridiculous that the big Q falls in mad love with some girl he's never even seen, but don't we all kind of think that our Aldonzas are Dulcineas? Like, if you're really into someone, you feel like you see them differently from the way other people see them. They seem extra special, and you really focus on their good qualities. We think it's funny that Don Quixote takes a burly farm girl like Aldonza to be a princess like Dulcinea, but how do we know—maybe she really is a Dulcinea inside.
We didn't think Sancho would make a good governor, either, but he actually pulled through in the end. Maybe Don Quixote isn't quite as crazy as he seems.
One of coolest things about Don Quixote is the way Cervantes uses humor and irony to simultaneously strip away our illusions and pretensions and suggest that there might be more to reality than we see on an everyday basis. The book is a total laugh fest, but it's also got kind of a romantic heart, at least some of the time.
(Note: Don Quixote's name appears in some editions as "Don Quijote." Don itself is not a name; it's a title that shows courtesy or respect, sort of like "Sir." Don Quixote translates to something like Sir Quixote.)