  Reid's theory of conception is at the heart of his philosophical system and his faculty psychology. The term ‘faculty psychology’ refers, at least for Reid, to both the distinction of certain belief-forming systems from others, and the explanatory utility purchased with such a set of distinctions. While faculty psychology appears to be an obvious way to scientifically study the senses, Reid's study of intellectual powers—memory, judgment, abstraction—under the umbrella of faculty psychology was not obvious. Reid would face pointed criticism for multiplying faculties, but this consequence was more than outweighed by the explanatory utility of separating mental powers—their inputs, their operations, and their outputs—from one another.