Everything you experience arrives already arranged. In space, in time, sorted into objects that persist, with some of them causing others.

You have never encountered an unarranged version to compare it with. So the question has an edge: how much of that arrangement is the world, and how much is the apparatus?

The reversal

Kant called his answer a Copernican turn, and the analogy is exact enough to be worth following.

Astronomy before Copernicus assumed the observer was fixed and the heavens moved, and produced ever more elaborate machinery to save the appearances. Copernicus moved the observer, and the appearances came out simply.

Philosophy before Kant assumed knowledge conforms to objects — the mind receives what is there and represents it accurately or badly. His proposal is to try it the other way: objects of experience conform to the mind's conditions for receiving them.

Space and time, on this account, are not containers the universe comes in. They are the forms in which anything can be given to a sensibility like ours. The categories — substance, causation, unity and the rest — are not patterns noticed in experience but conditions any experience must satisfy to be an experience of objects at all.

Which explains something otherwise inexplicable: how you can know in advance that every event you ever encounter will have a cause. Not because you have checked them all. Because a sequence you could not bring under causation would not show up as a sequence of events.

Empirically real, transcendentally ideal

The phrase is doing precise work and the doctrine is routinely mangled without it.

Empirically real. The table is there. It is there for everybody, it goes on being there when you leave, and physics describes it correctly. Nothing here dissolves the world into anybody's private impressions, and Kant thought the charge that it did was the worst misreading of his work.

Transcendentally ideal. The table as an object in space and time is the table as it must appear to a mind structured like ours. What it is independently of any conditions of experience is not something the question can reach.

Berkeley is the contrast Kant kept insisting on. To be is to be perceived makes existence depend on some particular mind doing the perceiving. Kant's dependence is structural rather than personal, and the world does not blink out when you stop attending to it.

He was worried enough about the confusion to add the Refutation of Idealism to the second edition in 1787, and its argument runs in the direction people expect least: consciousness of your own existence in time requires something permanent to measure that time against, which cannot itself be a passing state of yours, so it must be outside you. Inner experience presupposes outer reality.

The thing in itself

The most contested element, and the objection is old.

If things in themselves are beyond experience, on what grounds are they said to exist? Worse, Kant appears to say they cause our sensations — and causation is one of the categories, which he has just restricted to appearances. Jacobi put it best: you cannot get into the system without the thing in itself, and cannot stay in it with them.

Two readings have grown up around this. The two-world reading takes appearances and things in themselves to be numerically distinct, which makes the objection bite hard. The two-aspect reading takes them as one reality considered under two descriptions — as it is for us, and as it is apart from the conditions of our knowing — which blunts the objection and is closer to what most contemporary scholarship defends.

Either way, the negative use survives and is the more defensible: the thing in itself as a limit concept, marking where the questions stop being answerable rather than naming a hidden realm.

Why it did not stay in one place

Nearly every philosophical movement since has had to position itself against this.

German idealism took the reversal and removed the limit — if the mind constitutes the world of experience, why keep an unknowable remainder at all. Phenomenology inherits the method of asking what structures must hold for experience to be possible. And the whole family of positions holding that observation is theory-laden, that categories are conditioned by language or culture or history, is downstream of the same move, usually with Kant's fixed apparatus replaced by something contingent.

The recurring question is the one Kant left: if the frame is ours, can anything be said about what is framed? Every answer since has been a way of living with that.