QuestionsAre we free?You ought, therefore you can: the moral law reveals our freedom
Are we free?Free will

You ought, therefore you can: the moral law reveals our freedom

"Ought implies can" (transcendental freedom; ratio cognoscendi)

Immanuel Kant 1724 · 1804Set the limits of reason; act from duty.
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What it claims

We are free in the transcendental sense, from the practical point of view. We do not know it through science but through morality: the moral law is the reason for knowing freedom, and freedom is the reason for being of the moral law.

Why

We are conscious of an unconditional duty, and ought implies can: so we recognise ourselves as free.

The same idea through history
Immanuel Kant1781In the third antinomy, Kant shows that reason, when it looks for a first beginning, contradicts itself: the thesis says there must be a causality through freedom, the antithesis that everything happens by laws of nature. Science cannot decide.
Immanuel Kant1785In the Groundwork, Kant says we must think of ourselves as free in order to act, and that a rational being has two standpoints on himself: as a member of the world of sense (laws of nature) and as a member of the intelligible world (laws of reason).
Immanuel Kant1788In the Critique of Practical Reason, Kant reverses the order: freedom is not proved in order to ground morality. It is the moral law, of which we are conscious, that makes us know our freedom.
In short

"You ought, therefore you can: the moral law reveals our freedom" is an argument by Immanuel Kant (Critique of Practical Reason, 1788 (preface, §6); Critique of Pure Reason, 3rd antinomy, 1781) on the question "Are we free?" (answer: free will). Its claim: We are free in the transcendental sense, from the practical point of view. We do not know it through science but through morality: the moral law is the reason for knowing freedom, and freedom is the reason for being of the moral law. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P6). AskPhilosophy presents 3 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

Critique of Practical Reason, 1788 (preface, §6); Critique of Pure Reason, 3rd antinomy
Valid with an implicit premise

If everything in nature has a cause, how could we be free? Kant does not try to prove freedom by science: he starts from morality. When I know that I ought to do something, I judge that I can, even if I do not know whether I will. This awareness of duty makes me know a freedom within me that nothing else would show me. Kant draws a famous formula from it: the moral law is the reason for knowing (ratio cognoscendi) freedom, and freedom is the reason for being (ratio essendi) of the moral law. The phrase "you ought, therefore you can" sums up this reasoning; it is not a word-for-word quotation of Kant.

P1We are conscious of a moral law that commands unconditionally. This consciousness is a "fact of reason": it cannot be derived from anything else, for example from a prior consciousness of freedom. It imposes itself on us.
P2Whoever is conscious that he ought to do something judges that he can, and thus recognises in himself a freedom he would not know without the moral law. Kant shows this with two examples. A man who says he cannot resist his desire would resist it if a gallows awaited him right afterwards. And if his prince ordered him, on pain of death, to bear false witness against an honest man, he perhaps would not venture to say whether he would do it, but he admits without hesitation that it is possible for him.
P3This "can" presupposes freedom in the strong sense. If our acts are necessary consequences of past states, even when those causes are inner ideas and desires ("psychological" freedom), there is only natural necessity. We need a "transcendental" freedom, independent of everything empirical and of nature in general. Without it, no moral law and no moral imputation are possible.
P4We cannot start from freedom to reach the moral law: we are not immediately conscious of freedom, and experience shows us only the mechanism of nature, the opposite of freedom. We therefore start from the moral law, of which we are immediately conscious. It is the law that leads us to the concept of freedom.
P5Theoretical reason cannot prove freedom, but it can think it as not impossible. It needs freedom to escape the antinomy into which it falls when it looks for an absolute beginning in the series of causes (this is the third antinomy of the Critique of Pure Reason). It therefore does not refute freedom.
P6 · implicit premiseA condition of the moral law that theoretical reason does not refute may be affirmed as real from the practical point of view, without our understanding it for all that.
Therefore we are free in the transcendental sense, from the practical point of view. We do not know it through science but through morality: the moral law is the reason for knowing freedom, and freedom is the reason for being of the moral law.

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 P6, one can accept the other five premises and still refuse the conclusion: freedom may be necessary for morality and not refuted by science, but that does not prove it exists. A moralist could even conclude that morality is an illusion.
The argument is valid. P1 and P2 give the "I can" of duty, P3 says this power requires transcendental freedom, P5 says science does not refute it, and P6 draws the right to affirm it. P4 gives the direction of the move: we know freedom through the law, not the reverse. The points of attack are P1 (is the moral law a pure fact of reason?), P3 (does being able to do what we will not suffice?) and P6 (can we affirm what we do not understand?).
Analogy. Our image, not Kant's. The moral law is like a light switched on in a room: we do not see the door (freedom) directly, but the light shows it to us. Kant says the door really exists, and that without it the light could not have come on. The debate is about this last sentence.

Objections

3

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

Arthur Schopenhauer 1839 · attacks premise 1

Schopenhauer sets aside the reasoning "you can, because you ought", which he calls a "postulate" of Kant rather than a proof. Self-consciousness must not be confused with the moral conscience: moral instincts develop only after experience and reflection, and one cannot tell what is natural in them from what moral and religious education adds. So it would not be a pure and immediate fact of reason.

Kant replies
Kant 1788 · Kant's anticipated reply (1788)
The fact of the moral law is undeniable. One need only analyse the judgement people pass on the lawfulness of their actions: one always finds that, whatever inclination may say, their reason, incorruptible and self-constrained, relates the maxim of the will to the pure will, that is, to itself.
The debate goes on (our summary) Kant answers that the fact is found by analysing our moral judgements; Schopenhauer answers that these judgements are already shaped by education. The disagreement is sharp, but Schopenhauer is less far from Kant than he seems: in chapter 5 of the same essay he recalls Kant's solution (the intelligible and the empirical character) and judges it the finest and deepest thing humanity has produced. So he refuses the proof from morality but keeps the idea of a freedom that is not in the world of appearances. (our summary)
Pierre-Simon de Laplace 1814 · attacks premise 3

Laplace holds it to be an illusion that one determines oneself, without motives: one loses sight of the fleeting reasons for one's choice. We must regard the present state of the universe as the effect of its prior state and as the cause of the next. There is thus no room for a free causality in the chain of events.

Kant replies
Kant 1785 · Kant's anticipated reply (1785)
A rational being has two standpoints from which to regard himself and his actions. As a member of the world of sense, he is subject to laws of nature. As a member of the intelligible world, he is subject to laws independent of nature, founded in reason alone.
The debate goes on (our summary) Kant died ten years before Laplace's Essay. He would accept Laplace's determinism for phenomena: it is even the core of his position. His answer is that this determinism concerns only the world as it appears to us. Critics ask whether this solution saves freedom or merely moves it into a world we can neither know nor observe. (our summary)
David Hume 1748 · attacks premise 3

By liberty we can only mean a power of acting or not acting according to the determinations of the will: if we choose to remain at rest, we may; if we choose to move, we also may. This power, Hume says, which may be called hypothetical, belongs to everyone who is not a prisoner or in chains. So there is no subject of dispute.

Kant replies
Kant 1788 · Kant's reply to this kind of freedom (1788)
Reducing freedom to a comparative or psychological freedom is a wretched subterfuge, with which some still content themselves: they think they have solved with a petty word-play a problem on which centuries have laboured in vain.
Kant 1788 · Same passage, continued (1788)
If the freedom of our will were only this psychological and relative freedom, with nothing transcendental, it would be little better than the freedom of a turnspit, which, once wound up, performs its movements of itself.
The debate goes on (our summary) Kant does not name Hume here: his target is the freedom of Leibniz and his heirs, but it does aim at the same kind of freedom. Modern compatibilists answer that Kant asks too much: a freedom that escapes every cause would no longer be that of a responsible agent but mere chance, and the freedom morality and law need is precisely that of acting on one's reasons without constraint. (our summary)