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 [reason's] dictates. Every rational creature, it is 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, antient 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....
It is from the prospect of pain or pleasure that the aversion or propensity arises towards any object: And these emotions extend themselves to the causes and effects of that object, as they are pointed out to us by reason and experience. It can never in the least concern us to know, that such objects are causes, and such others effects, if both the causes and effects be indifferent to us. Where the objects themselves do not affect us, their connexion can never give them any influence; and it is plain, that as reason is nothing but the discovery of this connexion, it cannot be by its means that the objects are able to affect us.
Since reason alone can never produce any action, or give rise to volition, I infer, that the same faculty is as incapable of preventing volition, or of disputing the preference with any passion or emotion. This consequence is necessary. It is impossible reason could have the latter effect of preventing volition, but by giving an impulse in a contrary direction to our passion.... Nothing can oppose or retard the impulse of passion, but a contrary impulse.... We speak not strictly and philosophically when we talk of the combat of passion and of reason. Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.
Note here first of all that Hume describes the view that morality should be determined by reason as the dominant view in both ancient and modern philosophy. In other words, Rand is hardly the first philosopher to support a reason-based morality. Also note Hume's rejection of the traditional view that sees passion and reason as potentially at odds. No, contends Hume, they are not at odds. Passion (or emotion) serves as the motivation for reason. Passions provide the ends, and reason tells us the means by which those ends can be attained.
Hume's argument in the Treatise is a bit obscure to modern readers, so it helps to supplement it with passages from a later work. In Hume's An Inquiry Concerning the Principles of Morals, he explains with greater clarity the impossibility of a morality solely based on reason.
It is easy for a false hypothesis to maintain some appearance of truth, while it keeps wholly in generals, makes use of undefined terms, and employs comparisons, instead of instances. This is particularly remarkable in that philosophy, which ascribes the discernment of all moral distinctions to reason alone, without the concurrence of sentiment. It is impossible that, in any particular instance, this hypothesis [of a reason-only morality] can so much as be rendered intelligible; whatever specious figure it may make in general declamations and discourses. Examine the crime of ingratitude, for instance; which has place, wherever we observe good-will, expressed and known, together with good offices 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 good-will 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 the 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 actions 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 any thing abstruse here, you need only conclude, that your turn of mind is not suited to the moral sciences.
And Hume concludes as follows:
It appears evident, that the ultimate ends of human actions can never, in any case, be accounted for by reason, but recommend themselves entirely to the sentiments and affections of mankind, without any dependance on the intellectual faculties. Ask a man, why he uses exercise; he will answer, because he desires to keep his health. If you then enquire, why he desires health, he will readily reply, because sickness is painful. If you push your enquiries farther, and desire a reason, why he hates pain, it is impossible he can ever give any. This is an ultimate end, and is never referred to any other object.
Now as virtue is an end, and is desirable on its own account, without fee or reward, merely, for the immediate satisfaction which it conveys; it is requisite that there should be some sentiment, which it touches; some internal taste or feeling, or whatever you please to call it, which distinguishes moral good and evil, and which embraces the one and rejects the other... Reason, being cool and disengaged, is no motive to action, and directs only the impulse received from appetite or inclination, by showing us the means of attaining happiness or avoiding misery.
Rand either never read these passages in Hume or, if she did read them, she failed to understand them. If reason is perceived as a method by which matters of fact and relations are grasped and elucidated, how can a mere method generate a motive, let alone a moral end? Reason is but an intellectual tool; and since tools don't generate motives, but are merely methods or means by which some end is attained, something outside of reason, some appetite or passion or intention, must motivate reason to get it to do its thing. Rand's failure to grasp this led her to attempt the impossible: to create a morality based solely on "reason." And since, as Oakeshott reminds us, "to try to do something which is inherently impossible is always a corrupting enterprise," so Rand's so-called rational ethics terminates in what can only be described as a rationalistic moral code, where reason becomes a mere euphemism for casuistry.