If morality had naturally no influence on human passions and actions, it were in vain to take such pains to inculcate it; and nothing would be more fruitless than that multitude of rules and precepts, with which all moralists abound.  Philosophy is commonly divided into speculative and practical; and as morality is always comprehended under the latter division, it is supposed to influence our passions and actions, and to go beyond the calm and indolent judgments of the understanding.  And this is confirmed by common experience, which informs us, that men are often governed by their duties, and are detered from some actions by the opinion of injustice, and impelled to others by that of obligation.

Since morals, therefore, have an influence on the actions and affections, it follows, that they cannot be derived from reason; and that because reason alone, as we have already proved, can never have any such influence.  Morals excite passions, and produce or prevent actions.  Reason of itself is utterly impotent in this particular.  The rules of morality. therefore, are not conclusions of our reason.

No one, I believe, will deny the justness of this inference; nor is there any other means of evading it, than by denying that principle, on which it is founded.  As long as it is allowed, that reason has no influence on our passions and action, it is in vain to pretend, that morality is discovered only by a deduction of reason.  An active principle can never be founded on an inactive; and if reason be inactive in itself, it must remain so in all its shapes and appearances, whether it exerts itself in natural or moral subjects, whether it considers the powers of external bodies, or the actions of rational beings.

It would be tedious to repeat all the arguments, by which I have proved [Book II.  Part iii.  Sect 3.], that reason is perfectly inert, and can never either prevent or produce any action or affection. it will be easy to recollect what has been said upon that subject.  I shall only recall on this occasion one of these arguments, which I shall endeavour to render still more conclusive, and more applicable to the present subject.

Reason is the discovery of truth or falshood.  Truth or falshood consists in an agreement or disagreement either to the real relations of ideas, or to real existence and matter of fact.  Whatever, therefore, is not susceptible of this agreement or disagreement, is incapable of being true or false, and can never be an object of our reason.  Now it is evident our passions, volitions, and actions, are not susceptible of any such agreement or disagreement; being original facts and realities, compleat in themselves, and implying no reference to other passions, volitions, and actions.  It is impossible, therefore, they can be pronounced either true or false, and be either contrary or conformable to reason.