So says Maimonides:
It is clear, and proved by evidence, that moral virtues are a prerequisite of intellectual virtues. The attainment of true intellectual values, that is perfect intellectual ideas, is impossible except for a man whose moral character is properly trained and who possesses dignity and balance. There are many people whose native character and temper is wholly unsuitable for the attainment of perfection.
The Guide of the Perplexed, trans. Chaim Rabin (Indianapolis, IN: Hackett Publishing Co., 1952; orig. 1190), 61.