Nothing is more usual in philosophy, and even in common life, than to talk of the combat of passion and reason, to give the preference to reason, and assert that men are only so far virtuous as they conform themselves to its dictates. Every rational creature, 'tis said, is obliged to regulate his actions by reason; and if any other motive or principle challenge the direction of his conduct, he ought to oppose it, 'till it be entirely subdued, or at least brought to a conformity with that superior principle. On this method of thinking the greatest part of moral philosophy, ancient and modern, seems to be founded; nor is there an ampler field, as well for metaphysical arguments, as popular declamations, than this supposed pre-eminence of reason above passion. The eternity, invariableness, and divine origin of the former, have been displayed to the best advantage: the blindness, inconstancy, and deceitfulness of the latter, have been as strongly insisted on. In order to show the fallacy of all this philosophy, I shall endeavour to prove _first_, that reason alone can never be a motive to any action of the will; and _secondly_, that it can never oppose passion in the direction of the will. The understanding exerts itself after two different ways, as the judges from demonstration or probability; as it regards the abstract relations of our ideas, or those relations of objects of which experience only gives us information. I believe it scarce will be asserted, that the first species of reasoning alone is ever the cause of any action. As its proper province is the world of ideas, and as the will always places us in that of realities, demonstration and volition seem upon that account to be totally removed from each other. Mathematics, indeed, are useful in all mechanical operations, and arithmetic in almost every art and profession: but 'tis not of themselves they have any influence. Mechanics are the art of regulating the motions of bodies _to some designed end or purpose_; and the reason why we employ arithmetic in fixing the proportions of numbers, is only that we may discover the proportions of their influence and operation. A merchant is desirous of knowing the sum total of his accounts with any person: why? but that he may learn what sum will have the same _effects_ in paying his debt, and going to market, as all the particular articles taken together. Abstract or demonstrative reasoning, therefore, never influences any of our actions, but only as it directs our judgment concerning causes and effects; which leads us to the second operation of the understanding.