The book gives no name for an illness. What the text says, right in chapter I, is that the gentleman, from little sleep and much reading, had his brain dried out and lost his judgment. The rest are observations by those around him, and none is a diagnosis.
The narrator gives the cause: nights and days of reading books of chivalry, until he took those inventions for truth and decided to live as the knights did. The madness has an object and a method. He practices it with firmness, has a ready explanation for every refutation, an enemy magician who turns giants into windmills, and in the Sierra Morena he performs the penance of Beltenebros for no reason, on purpose, because he wants to imitate Amadis.
Those who meet him give him different names. The gentleman Don Lorenzo, son of Don Diego de Miranda, calls him a madman of mixed sorts, full of lucid intervals; the churchman at the dukes' table calls him a fool; the bachelor Sansón Carrasco wants to cure him; the squire Tomé Cecial asks which madman is madder, the one who is so because he cannot help it or the one who is so by his own will. The bachelor answers that the first will always be so, and the second will stop being so when he wishes.
At the end, on waking from a fever, the gentleman himself says that he has free and clear judgment, without the shadows of ignorance that the bitter and continual reading of the detestable books of chivalry cast upon him. He asks that death not confirm the reputation for madness, for, even though he was mad, he did not want to confirm it. The book does not diagnose him; it leaves open the question of who decides when someone is mad.