QuestionsShould we fear death?Immortality as a postulate of practical reason
Should we fear death?The soul survives

Immortality as a postulate of practical reason

The postulate of the immortality of the soul

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

The immortality of the soul is a postulate of practical reason.

Why

The moral law commands holiness, reachable only through endless progress, which requires an endless existence.

In short

"Immortality as a postulate of practical reason" is an argument by Immanuel Kant (Critique of Practical Reason, 1788 (5:122-124)) on the question "Should we fear death?" (answer: the soul survives). Its claim: The immortality of the soul is a postulate of practical reason. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4). AskPhilosophy presents 3 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Critique of Practical Reason, 1788 (5:122-124)
Valid with an implicit premise

Kant does not claim to prove that the soul survives. He starts from morality: the moral law asks us to be perfectly good, which no human being can become in one life. For the demand not to be absurd, one must be able to progress without end, so to exist without end. Immortality is not knowledge: it is what practical reason must assume.

P1The moral law commands us to realise the highest good, and its supreme condition is the complete accordance of our will with the law.
P2What the law commands must be possible: this complete accordance must therefore be attainable.
P3This complete accordance is holiness. No rational being in the sensible world is capable of it at any moment of its existence.
P4 · implicit premiseThere is no other way to get there than the person's own progress, approaching holiness without end (neither instant transformation nor help from elsewhere).
P5An endless progress is possible only if the same person exists without end.
P6What is inseparably tied to the moral law and necessary to the possibility of its object is a postulate of pure practical reason: a theoretical proposition that cannot be demonstrated, but must be accepted in order to act.
Therefore the immortality of the soul is a postulate of practical reason.

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 P4, the computer finds a case where holiness is possible, nobody reaches it at any given moment, and yet no endless progress is required.
Without P4, nothing says that the only road to holiness is an infinite progress: outside help or a sudden change could be enough, and immortality would no longer be needed. Kant sets these possibilities aside here without discussing them (in Religion within the Bounds of Mere Reason he makes room for grace).
Analogy. You are asked to run a marathon of infinite length, and told it is possible. If that is true, you must be able to run without end. Kant says the same of perfect goodness: if the command makes sense, an endless life is needed to approach it.

Objections

3

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

Kant himself (Critique of Pure Reason) 1787 · attacks the conclusion

In the preface to the second edition, Kant writes that he cannot assume God, freedom and immortality for the needs of morality unless he strips speculative reason of its claims to insights that go beyond experience. He had to abolish knowledge to make room for belief. In the "Transcendental Dialectic", the paralogisms of pure reason also show that the proofs of a simple and immortal soul are not valid. Immortality therefore cannot be known.

Kant replies
Kant (Critique of Practical Reason) Kant's reply in the Critique of Practical Reason
A postulate of pure practical reason is a theoretical proposition which, as such, cannot be demonstrated, but which is inseparable from a practical law valid a priori.
The debate goes on (our summary) Our summary: Kant therefore accepts the objection. It is not knowledge but an "assent" necessary for whoever recognises the moral law. What remains is whether practical reason can justify a belief that theoretical reason cannot. The paralogisms are not in our corpus, so we cite them by their place in the work (A341-405 / B399-432).
Where the debate stands (our summary)Kant and his critics agree on one point: immortality cannot be proved. The core disagreement concerns whether a belief grounded only on moral necessity is legitimate. An open point, our summary.
Ludwig Feuerbach 1841 · attacks the conclusion

In The Essence of Christianity, Feuerbach argues that belief in personal immortality is identical to belief in a personal God: both express man's wish to have no limits. Kant's postulate therefore does not reveal a demand of reason, it gives shape to a human wish (see the argument "Religion as man's projection").

no reply
No reply possible: objection raised in 1841, after the author's death (1804).
Where the debate stands (our summary)Kantians reply that Kant does not start from a wish to survive but from a demand of the moral law, which is severe towards us. For Feuerbach that demand itself comes from human nature. The core disagreement is whether the psychological origin of a belief tells us anything about its truth. An open point, our summary.
Arthur Schopenhauer 1840 · attacks premise 1

Schopenhauer judges that Kant's "unconditional" obligation hides conditions: it presupposes someone who rewards, a reward, and the immortality of the person rewarded. Kant's ethics would be a disguised theology, which his postulates bring out into the open.

no reply
No reply possible: objection raised in 1840, after the author's death (1804).
Where the debate stands (our summary)Kant's defenders reply that the postulate of immortality rests on the holiness to be reached and not on a reward, and that the moral law commands without promising anything. Schopenhauer answers that an obligation only makes sense through a threat or a promise. An open point, our summary.