QuestionsDoes time really exist?Time is the form of our inner sense: real for experience, nothing in itself
Does time really exist?Time is in the mind

Time is the form of our inner sense: real for experience, nothing in itself

The transcendental ideality of time; time as the a priori form of inner sense

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

Time is the form of our inner sense, the a priori condition of all intuition. It has empirical reality (it holds for everything we can experience) and transcendental ideality (apart from our way of intuiting, it is nothing).

Why

Time is not drawn from experience, it makes experience possible; so it must be a form of our mind, not a thing in itself.

The same idea through history
Augustine of Hippo400Augustine already places time in the soul (distension of the mind), but as an extension that the soul measures. Kant makes it a condition of all experience.
Immanuel Kant1787Kant: time is not a concept drawn from experience, it is the a priori form of inner intuition, hence of all possible experience.
In short

"Time is the form of our inner sense: real for experience, nothing in itself" is an argument by Immanuel Kant (Critique of Pure Reason, Transcendental Aesthetic, §§ 4-7 (1781, 2nd ed. 1787), 1787) on the question "Does time really exist?" (answer: time is in the mind). Its claim: Time is the form of our inner sense, the a priori condition of all intuition. It has empirical reality (it holds for everything we can experience) and transcendental ideality (apart from our way of intuiting, it is nothing). According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P5). AskPhilosophy presents 4 objections, 4 of them with a documented reply, and the original text behind each premise, checked word for word.

Critique of Pure Reason, Transcendental Aesthetic, §§ 4-7 (1781, 2nd ed. 1787)
Valid with an implicit premise

For Newton, time is a great river that flows of itself, whether things are there or not. For Leibniz, it is only an order between things. Kant decides otherwise: we cannot perceive anything without placing it in time, and we know with certainty some things about time (it has one dimension, its moments follow one another) that experience could not teach us. So time comes from us: it is the form of our inner sense, as space is that of our outer senses. It is real for everything we can experience, but it is not a property of things in themselves.

P1Time is not a concept drawn from experience. We could not even perceive that things are simultaneous or successive if the idea of time were not already there.
P2Time is a necessary representation: we can remove phenomena from time in thought, but we cannot remove time from phenomena.
P3We have certain principles about time (one dimension; different times are successive, not simultaneous). Experience gives neither strict universality nor absolute certainty: these principles therefore do not come from it.
P4If time existed by itself, or as an order inherent in things, it could not precede objects as their condition, and we could not know it a priori by synthetic propositions.
P5 · implicit premiseThere are only three possibilities: time is a thing in itself, or an order of things in themselves, or the subjective condition of our intuition. Moreover, what is known a priori (P1 to P3) could not have been learned from things.
So time is the form of our inner sense, the a priori condition of all intuition. It has empirical reality (it holds for everything we can experience) and transcendental ideality (apart from our way of intuiting, it is nothing).

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 P5, the computer finds a case where time is known a priori and yet is not only in the subject: it would be both in things and in our intuition, or we would know it because our mind is made for things as they are.
Valid with P5, the hidden premise: the only possibilities are the three listed, and what is known a priori does not come from things. P1 to P3 show that we know time a priori; P4 rules out that time is in things; P5 concludes that it is in the subject. Critics have mostly targeted this disjunction (see Trendelenburg).
Analogy. Wear green-tinted glasses: everything you see is green, and you can say in advance that everything you will see is. This certainty tells you nothing about the objects, it comes from your glasses. For Kant, time is like those glasses: it colours all our experience, and we can say in advance that it will be in time.

Objections

4

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

J
Johann Heinrich Lambert and Moses Mendelssohn 1770 · attacks the conclusion

Changes are real: the succession of our own representations proves it, even if one denies all external phenomena. Now changes are only possible in time. So time is something real, and not merely a form of our mind.

Kant replies
Immanuel Kant 1787 · Kant's reply
Kant grants the whole argument: time is something real, as the real form of inner intuition. It therefore has reality in relation to inner experience. What he refuses is that time be an object or that it have an absolute reality, independent of our way of intuiting.
Where the debate stands (our summary)Our summary: Kant does not deny that change is real for us; he denies that time is anything other than the form in which it appears to us. Kant does not name his opponents in the Critique (he speaks of 'penetrating minds'); they are identified from their 1770 correspondence. The core disagreement remains: can something be called 'real' without being real in itself?
Isaac Newton 1687 · attacks premise 4

Absolute, true and mathematical time, of itself and from its own nature, flows equably without relation to anything external. Clocks and motions give only approximate measures of it.

a defender replies
see the argument
We have not yet found a direct reply by Kant in the texts.
Immanuel Kant defender of the argument · 1787
Kant charges those who make time a substance (the party of 'mathematical natural philosophers') with admitting two eternal and infinite nonentities, which exist without anything real being there, only to contain everything real.
The debate goes on (our summary) Kant writes after Newton and targets those who make space and time 'substances' ('mathematical natural philosophers') without naming him. He charges them with admitting 'two eternal, infinite nonentities that exist only to contain everything real'.
Where the debate stands (our summary)Our summary: this is the classic debate between a 'container' time (Newton) and a time tied to the observer (Kant) or to the order of things (Leibniz). The first explains why physics works, the second why we know some things about time in advance. A disputed point.
A
Adolf Trendelenburg 1840 · attacks premise 5

Kant has not shown that time cannot be both a form of our intuition and a property of things. That time comes from us does not prevent it from also corresponding to what is. Kant's disjunction therefore has a hole, which has since been called 'Trendelenburg's gap'.

a defender replies
No reply possible: objection raised in 1840, after the author's death (1804).
K
Kuno Fischer defender of the argument · 1870
Kuno Fischer defends Kant against Trendelenburg in the Anti-Trendelenburg (1870). He maintains that Kant had already settled this point.
Where the debate stands (our summary)Our summary: the quarrel between Trendelenburg and Fischer was fierce in the 19th century and turns on one point: does the a priori suffice to exclude that time is also in things? An open point: Kantians and their critics have not agreed.
Albert Einstein 1905 · attacks premise 3

For Kant, all times are parts of one and the same time, valid for everyone. But Einstein shows in 1905 that two events that are simultaneous for one observer are not for another who is moving: there is no single time for all, and what Kant held certain in advance depends on experience (on the speed of light).

a defender replies
see the argument
No reply possible: objection raised in 1905, after the author's death (1804).
E
Ernst Cassirer defender of the argument · 1921
Cassirer argues that relativity does not refute Kant. What Kant holds a priori is that all experience is arranged in a temporal order, not a precise way of measuring time. Physics changes the measure, not the requirement of an order.
Where the debate stands (our summary)Our summary: Einstein did not write against Kant; his commentators drew this objection from his theory. The core disagreement is about what Kant held a priori: a precise structure (a single time) or only the general form of an order. A disputed point.
What today's voices say
Defend it
Donald HoffmanHe agrees with Kant that we do not see nature as it is and that space and time are forms of our perception, but says Kant was wrong to think Euclidean space-time is a priori.