Yes, I understand what they believe, meaning the same thing as I understand why they believe this. — Terrapin Station
Lolwut? — Sapientia
Unless we're positing and talking about underlying psychological motivations that the person doesn't explicitly connect with the belief(s) in question, why someone believes something is part of what they believe, no? So if you understand what they believe about x, then you understand why they believe it.
Murray believes that his car is parked on Main Street, because when he returned home yesterday, he parked on Main Street and he has no reason to believe that anyone moved his car--he was parked in a legal space, no one else has access to his car, etc., and chances are that it's not stolen.
That's all what
Murray believes, and it includes why
he believes it. Why Murray believes what he does is part of what he believes. "When I returned home yesterday I parked on Main Street," "I was parked in a legal space," etc. are all beliefs, and they're the supporting beliefs for "My car is parked on Main Street." Understanding what he believes includes understanding all of that--otherwise you don't understand
what he believes.
Now, if we're wondering about underlying psychological reasons that Murray believes that when he returned home from work yesterday, he parked on Main Street that's another issue. And this is especially pertinent when there is reason to think that Murray doesn't have good reasons to believe what he does, as in that case, some people who are familiar with what Murray believes might not know offhand the underlying psychological reasons simply by understanding what he believes. For example, maybe Murray is actually institutionalized, and he has no job, no car, etc., so that the underlying reasons that Murray believes what he does is that he's highly delusional, etc. For that sort of knowing why someone believes what they do, we need to have more detailed personal information about them, so that we have some clues about their psychological dispositions, quirks, etc. But that's probably not what was meant in the case at hand.