T 3.1.1.18

If the thought and understanding were alone capable of fixing the boundaries of right and wrong, the character of virtuous and vicious either must lie in some relations of objects, or must be a matter of fact, which is discovered by our reasoning. This consequence is evident. As the operations of human understanding divide themselves into two kinds, the comparing of ideas, and the inferring of matter of fact; were virtue discover'd by the understanding; it must be an object of one of these operations, nor is there any third operation of the understanding, which can discover it. There has been an opinion very industriously propagated by certain philosophers, that morality is susceptible of demonstration; and tho' no one has ever been able to advance a single step in those demonstrations; yet 'tis taken for granted, that this science may be brought to an equal certainty with geometry or algebra. Upon this supposition, vice and virtue must consist in some relations; since 'tis allow'd on all hands, that no matter of fact is capable of being demonstrated. Let us, therefore, begin with examining this hypothesis, and endeavour, if possible, to fix those moral qualities, which have been so long the objects of our fruitless researches. Point out distinctly the relations, which constitute morality or obligation, that we may know wherein they consist, and after what manner we must judge of them.