From James Rachels, "The Right Thing to Do: Basic Readings in Moral Philosophy", 2nd ed. (New York: McGraw Hill, 1999), pp. 60-64.
Excerpted from David Hume, A Treatise of Human Nature (1740), bk. 3, pt. 1, sec. 1; and An Inquiry Concerning the Principles of Morals (1751), app. 1.
Those who affirm that virtue is nothing but a conformity to reason; that there are eternal fitnesses and unfitnesses of things, which are the same to every rational being that considers them; that the immutable measures of right and wrong impose an obligation, not only on human creatures, but also on the Deity himself: All these systems concur in the opinion, that morality, like truth, is discern'd merely by ideas, and by their juxta-position and comparison. In order, therefore, to judge of these systems, we need only consider, whether it be possible, from reason alone, to distinguish betwixt moral good and evil, or whether there must concur some other principles to enable us to make that distinction.
If morality had naturally no influence on human passions and actions, 'there in vain to take such pains to inculcate it; and nothing wou'd 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, 'tis supposed to influence our passions and actions, and to go beyond the calm and indolent judgments of the understanding. And this is confirm'd by common experience, which informs us, that men are often govern'd by their duties, and are deter'd from some actions by the opinion of injustice, and impell'd to others by that of obligation.
Since morals, therefore, have an influence on the actions and affections, it follows, that they cannot be deriv'd from reason; and that because reason alone, as we have already prov'd, 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.
... Take any action allow'd to be vicious: Wilful murder, for instance. Examine it in all lights, and see if you can find that matter of fact, or real existence, which you call vice. In whichever way you take it, you find only certain passions, motives, volitions and thoughts. There is no other matter of fact in the case. The vice entirely escapes you, as long as you consider the object. You never can find it, till you turn your reflexion into your own. breast, and find a sentiment of disapprobation, which arises in you, towards this action. Here is a matter of fact; but 'tis the object of feeling, not of reason. It lies in yourself, not in the object. So that when you pronounce any action or character to be vicious, you mean nothing, but that from the constitution of your nature you have a feeling or sentiment of blame from the contemplation of it.
... I cannot forbear adding to these reasonings an observation, which may, perhaps, be found of some importance. In every system of morality, which I have hitherto met with. I have always remark'd, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surpriz'd to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it shou'd be observ'd and explain'd; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others; which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention wou'd subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceiv'd by reason....
Examine the crime of ingratitude, for instance, which has place wherever we observe good-will expressed and known, together with goodoffices performed, on the one side, and a return of ill-will or indifference with ill-offices or neglect on the other: anatomize all these circumstances and examine, by your reason alone, in what consists the demerit or blame. You never will come to any issue or conclusion.
Reason judges either of matter of fact or of relations. Enquire then, first, where is that matter of fact which we here call crime, point it out, determine the time of its existence, describe its essence or nature, explain the sense or faculty to which it discovers itself It resides in the mind of the person who is ungrateful. He must, therefore, feel it and be conscious of it. But nothing is there, except the passion of ill-will or absolute indifference. You cannot say that these, of themselves, always and in all circumstances are crimes. No, they are only crimes when directed towards persons who have before expressed and displayed goodwill towards us. Consequently, we may infer that the crime of ingratitude is not any particular individual fact, but arises from a complication of circumstances which, being presented to the spectator, excites the sentiment of blame by the particular structure and fabric of his mind.
This representation, you say, is false. Crime, indeed, consists not in a particular fact, of whose reality we are assured by reason, but it consists in certain moral relations, discovered by reason, in the same manner as we discover by reason the truths of geometry or algebra. But what are the relations, I ask, of which you here talk? In the case stated above, I see first good-will and good-offices in one person, then ill-will and ill-offices in the other. Between these, there is a relation of contrariety. Does the crime consist in that relation? But suppose a person bore me ill-will or did me ill-offices, and I, in return, were indifferent towards him, or did him good offices. Here is the same relation of contrariety, and yet my conduct is often highly laudable. Twist and turn this matter as much as you will, you can never rest the morality on relation, but must have recourse to the decisions of sentiment.
When it is affirmed that two and three are equal to the half of ten, this relation of equality I understand perfectly I conceive that, if ten be divided into two parts, of which one has as many units as the other, and if any of these parts be compared to two added to three, it will contain as many units as that compound number. But when you draw thence a comparison to moral relations, I own that I am altogether at a loss to understand you. A moral action, a crime, such as ingratitude, is a complicated object. Does the morality consist in the relation of its parts to each other? How? After what manner? Specify the relation: be more particular and explicit in your propositions, and you will easily see their falsehood.
No, say you, the morality consists in the relation of actions to the rule of right; and they are denominated good or ill, according as they agree or disagree with it. What then is this rule of right? In what does it consist? How is it determined? By reason, you say, which examines the moral relations of actions. So that moral relations are determined by the comparison of action to a rule. And that rule is determined by considering the moral relations of objects. Is not this fine reasoning?
All this is metaphysics, you cry. That is enough; there needs nothing more to give a strong presumption of falsehood. Yes, reply I here are metaphysics surely; but they are all on your side, who advance an abstruse hypothesis which can never be made intelligible, nor quadrate with any particular instance or illustration. The hypothesis which we embrace is plain. It maintains that morality is determined by sentiment. It defines virtue to be whatever mental action or quality gives to a spectator the pleasing sentiment of approbation; and vice the contrary. We then proceed to examine a plain matter of fact, to wit, what actions have this influence. We consider all the circumstances in which these actions agree, and thence endeavour to extract some general observations with regard to these sentiments. If you call this metaphysics and find anything abstruse here, you need only conclude that your turn of mind is not suited to the moral sciences.