Tomas wakes in the middle of the night and realizes that he had a series of erotic dreams, and the last one was of an extremely overweight woman in a pool. Tomas wonders why he was excited by a woman who would repulse him in real life. The human brain is like two cogwheels, Tomas decides, with images on one and the body’s reactions on the other. The naked woman image corresponds with the erection cog, but the wheel can get knocked out of sync. The erection cog can correspond with a swallow, for instance, and then the sight of a bird would cause excitement. So, if Tomas’s cogwheel was knocked out of sync and he got an erection looking at a bird, it would have nothing to do with his love for Tereza. Equating love with sex is one of the Creator’s strangest ideas, Tomas thinks.
This passage more thoroughly explains Tomas’s understanding of sex and love, which frames them as completed unrelated and quite arbitrary. With this understanding of sex and love, Tomas is able to completely detach love from sex, which Tereza finds impossible to understand. But while Tomas is able to sleep with women without loving them, he’s also starting to find that he can’t separate his love for Tereza from these other sexual encounters, which is why he must drink alcohol before he can have sex with other women. In other words, Tomas tells himself that sex and love are unrelated, but his actions suggest otherwise.