Psychosis can mess with critical thinking skills and make completely irrational things seem perfectly logical at times. When a delusion kicks in, it can be very difficult to logic your way out of it.
I periodically think my milk is filled with slugs and larvae, even though I know rationally, that's not the case. Most days, I can work through that by reminding myself that I live with other people who also drink the milk, and if there were bugs in it, someone else would have noticed, so it's more logical that I'm just hallucinating.
However, the worse psychosis gets, the ability to logic like that can become more difficult. I might start thinking things like "they're only pretending to drink the milk to trick me" or something. They're not, and the milk is fine, but... the brain fills in the gaps of the logic with whatever it can to preserve the delusion.
Delusions often sound completely ridiculous on the outside, but when you're in the delusional state, it feels completely rational, often in ways you can't fully explain.
I often come out of delusions and think "how did I think that was real", but… well, brains are mean like that, y'know?
