QuestionsWhat makes something right?Reason is the slave of the passions: morality is felt, not demonstrated
What makes something right?Nature or conscience

Reason is the slave of the passions: morality is felt, not demonstrated

"Reason is the slave of the passions"; Hume's law (no "ought" from "is")

David Hume 1711 · 1776Neither causation nor induction can be proved: custom guides us.
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What it claims

Morality does not come from reason: it is felt rather than judged.

Why

Reason alone never moves us to act, yet morality does move us to act: so it must come from feeling.

The same idea through history
Francis Hutcheson1725Hutcheson, Hume's mentor, holds that we perceive right and wrong through a "moral sense": we judge differently two people who do us the same service, one out of kindness, the other out of self-interest.
David Hume1739Hume, aged 28, goes further: reason alone cannot make us do anything, it only serves our passions. His striking formula: it is not contrary to reason to prefer the destruction of the whole world to the scratching of one's finger. So morality, which makes us act, comes from feeling.
Adam Smith1759Adam Smith, Hume's friend, builds a whole ethics on sympathy: we judge actions by putting ourselves, in imagination, in other people's place.
In short

"Reason is the slave of the passions: morality is felt, not demonstrated" is an argument by David Hume (A Treatise of Human Nature, Book 2, 3.3 and Book 3, 1.1-2 (1739-1740), 1739) on the question "What makes something right?" (answer: nature or conscience). Its claim: Morality does not come from reason: it is felt rather than judged. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

A Treatise of Human Nature, Book 2, 3.3 and Book 3, 1.1-2 (1739-1740)
Valid with an implicit premise

When you judge that murder is wrong, is it your reason that proves it, as in mathematics? For Hume, no. Reason serves to know what is true or false, to find the means to reach a goal, but it never moves us to act: our passions do. Yet morality moves us to act. So it does not come from reason alone: we feel right and wrong more than we judge them. In the same chapter, Hume adds a remark that became famous as "Hume's law": from what is, one cannot by itself deduce what ought to be.

P1Reason alone can never move us to act nor oppose a passion: it is, and ought only to be, the slave of the passions.
P2Yet morality acts on us: it excites passions, it produces or prevents actions.
P3 · implicit premiseWhat produces an effect that reason alone cannot produce cannot come from reason alone.
P4Our moral judgements are perceptions of the mind, and every perception is either an idea (the business of reason) or an impression (a feeling).
P5An example confirms it: examine a murder from every angle, you find only passions, motives, thoughts. The vice appears only when you look into yourself and find there a feeling of disapproval.
P6Likewise, no reasoning about what "is" allows us, by itself, to conclude to what "ought" to be: the "ought" expresses a new relation, which would need explaining. This is "Hume's law".
Therefore morality does not come from reason: it is felt rather than judged.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a case where reason alone never moves us to act, where morality does, and where morality nevertheless comes from reason: it is enough for moral reason to be an exception to the rule in P1.
The core of the argument is P1, P2, P3 and P4, and it is valid. Without P3 it no longer is: it would remain possible for something to produce an effect that reason alone does not produce and still come from reason, for instance if reason could sometimes, by itself, become a motive. That is exactly what Kant will claim. P5 and P6 are a second, independent proof: the murder example and Hume's law show that right and wrong are not found in the observed facts. They strengthen the conclusion but are not needed for validity.
Analogy. A GPS can tell you which road leads fastest to the beach. It cannot tell you whether you should go to the beach: your desire decides that. For Hume, reason is the GPS, and morality comes from the side of desire, not from the GPS.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Kant 1785 · attacks premise 3

A moral law must hold with absolute necessity, for every rational being, not only for humans. Its ground must therefore be sought not in human nature, its feelings or its circumstances, but a priori in pure reason. A rule based on experience can be a practical rule, never a moral law.

Hume replies
Hume What Hume had already answered to the rationalists of his day (Kant wrote after his death)
Morality has an influence on our actions and passions; reason alone has none. So the rules of morality are not conclusions of our reason.
Kant Kant's answer (Critique of Practical Reason, 1788)
Pure reason is practical of itself alone: it gives man a universal law, the moral law. So it can move us to act without the help of any passion.
The debate goes on (our summary) The whole debate turns on P3: Hume says reason cannot be a motive, Kant says it can. Kant does grant that a feeling comes in, respect for the law, but he says it is produced by reason itself. Hume's heirs reply that this respect is precisely a passion. The disagreement remains lively today between "Humeans" and "Kantians".
Thomas Reid 1788 · attacks the conclusion

If morality is only a feeling, it is neither true nor false, and each person has their own. Yet when I judge that one ought not to steal, or kill, or bear false witness, I am as sure of it as of a theorem of Euclid. I judge, I do not merely feel; and a judgement can be true or false, a feeling cannot.

Hume replies
Hume Hume's answer to the "each to their own" worry (written before Reid): the general point of view
Each person sees others from their own place, and our feelings vary with distance. To avoid endless contradictions and reach a more stable judgement, we all place ourselves at steady and general points of view, whatever our present situation.
Hume Hume returns to it in the Enquiry concerning the Principles of Morals (1751)
When we call someone a rival, we speak the language of self-love. But when we call them vicious or odious, we expect everyone to agree: we must leave our particular situation and choose a point of view common to us and others, that of the feeling of humanity all people share.
The debate goes on (our summary) The general point of view explains why people largely agree in morals and can argue reasonably. But it does not fully answer Reid: a stable agreement between feelings is not yet a truth. Reid would reply that if everyone agreed to approve of cruelty, Hume would have nothing left to object.
John Searle 1964 · attacks premise 6

One can derive an "ought" from an "is". Jones uttered the words "I hereby promise to pay you five dollars": that is a fact. So he promised; so he placed himself under an obligation to pay; so he ought to pay. Each step rests on facts and on the meaning of words, with no hidden value judgement.

no reply
No reply possible: objection raised in 1964, after the author's death (1776).
The debate goes on (our summary) Searle's critics, R. M. Hare as early as 1964, answer that the step "he promised, so he ought to pay" assumes that one accepts the institution of promising, that is, a rule: promises must be kept. The "ought" was thus already hidden in the starting point. Searle answers that this rule is part of the very meaning of the word "promise". The debate is about whether an institutional fact is still a mere fact.
What today's voices say
Defend it
Jordan PetersonHe says he buys Hume's argument that you cannot derive an ought from an is: what you should do is not the same as what is.
Steven "Destiny" BonnellAgainst his opponent he says they have "jumped over an is-ought thing": preferences do not make a moral fact.
Criticize it
Sam HarrisIn 2024 he rejects Hume's view that moral truths can only move us when joined with a passion, calling it an unrealistic picture of the human mind.
Alex O'ConnorAgainst Sam Harris's moral landscape he says it commits the is-ought fallacy.