Earlier quoted context omitted.
《SPOILERS 》There is some idea that if conceived can destroy you. Greco wants to use it to attack the protagonist. The protagonist defends himself by delaying his perception and passing all inputs through a simulated concept so that when Greco communicates the idea it would destroy the sandbox and not himself. But Greco never tells the protagonist the idea. It just lets the protagonist formulate it themselves as a rea…
The protagonist is super intelligent but didn't foresee this? Also, what is the idea that destroys him?
It's unspecified and supposed to be unique for everyone. I think from the initial derision the protagonist doesn't believe in the concept.