From the detour road sign “unlike,” you can tell that Hume isn’t trying to determine whether an objective reality exists. Why wouldn’t he do so? Following this logic reveals the right word for the sentence. Hume disregarded objective reality not because he thought the issue was (A) pragmatic, “practical,” or (B) challenging. That would have made him want to investigate it. He probably didn’t try because he felt that the issue was either unverifiable or uninteresting. Choice (D) insoluble, “not capable of being solved,” fits the blank best. It’s unclear that Hume would be “unlike most other philosophers” if he thought the issue was (C) theoretical or (E) esoteric (“understood only by a select few”), so both are incorrect.