Tomas wakes up in the middle of the night from a series of erotic dreams.
In the last dream, he was incredibly excited at the thought of making love to an obese woman floating on her back in a swimming pool, covered in hair.
He wakes up wondering how he could be sexually excited by an image like that, especially when his stomach felt so horrible.
He decides that there are two wheels turning in the brain. One shows images, and the other dictates the corresponding reactions from the body. The wheels must have gotten out of sync.
The narrator points out that Tomas having an erection at the sight of another woman has no bearing on his love for Tereza.
Perhaps our Creator uses excitement as an amusement for himself, but love belongs to us and us alone. Love is our freedom; love lies beyond es muss sein.
No, says the narrator, that can't be strictly true. Love is somewhat attached to the clockwork running in our brains that dictates sexual attraction.
And that's what is so bizarre.
Attaching love to sexual excitement, thinks Tomas, is the most bizarre idea that our Creator has ever had.
Then, as he begins to fall back to sleep, Tomas has a revelation: all he has to do is attach sexual arousal to something trivial, like the sight of a swallow, and then he can love Tereza without being disturbed by sex.
He thinks he has found the key to all mysteries, the ultimate solution. And then he falls asleep.