This question is motivated by discussions like the following that I've seen/had from time to time:
Jane: "X believed it would eventually become a danger and that doing this-or-that would stop the danger, which explains why X did this-or-that."
Bill: "But it wouldn't become a danger. And doing this-or-that wouldn't have worked."
Jane: "I didn't say it would become a danger or that this-or-that would work. I said X believed these things to be true, which is why X did this-or-that."
Bill: "It doesn't matter what X believes. It would never be a danger and he shouldn't have done this-or-that."
Bill appears to be unable to understand X's reasoning.
If it had been X's emotions that Bill wasn't able to understand then we'd say that Bill lacked empathy. But since it's instead X's reasoning that Bill can't understand, we say that Bill lacks _____ (what)?