Maimonides on Becoming a Prophet

"It is impossible that an ignorant person should be a prophet; or that a person being no prophet in the evening, should, unexpectedly on the following morning, find himself a prophet, as if prophecy were a thing that could be found unintentionally.

"But if a person, perfect in his intellectual and moral faculties, and also perfect, as far as possible, in his imaginative faculty, prepares himself in the manner which will be described, he must become a prophet; for prophecy is a natural faculty of man. 

"It is impossible that a man who has the capacity for prophecy should prepare himself for it without attaining it, just as it is impossible that a person with a healthy constitution should be fed well, and yet not properly assimilate his food, and the like."

-- Maimonides, Moreh Nevuchim 2:32