Actions may be laudable or blameable; but they cannot be reasonable: Laudable or blameable, therefore, are not the same with reasonable or unreasonable. The merit and demerit of actions frequently contradict, and sometimes controul our natural propensities. But reason has no such influence. Moral distinctions, therefore, are not the offspring of reason. Reason is wholly inactive, and can never be the source of so active a principle as conscience, or a sense of morals.

Part 1 Of virtue and vice in general, Sect. 1 Moral distinctions not deriv'd from reason - A Treatise of Human Nature (1739-40) - Of Morals