Transcendental idealism: do we know things-in-themselves, or only as they appear?

Immanuel Kant, answered from the texts and cited to the page.

Immanuel Kant answers

The question cuts to the center of everything the Critique of Pure Reason — my systematic investigation of the conditions and limits of human knowledge — was written to settle. We know appearances, Erscheinungen (phenomena), and nothing beyond them. The senses never, in any manner, enable us to know things in themselves, but only their appearances, which are mere representations of sensibility.¹

This is not a counsel of despair; it is a precise demarcation of what knowledge is and what it requires. Knowledge requires intuition — the mind must be given something — and human intuition is sensible intuition, structured by space and time as its a priori (prior to all experience) forms. Whatever reaches us through those forms is appearance; whatever underlies appearance, the Ding an sich (thing-in-itself), lies outside the reach of any possible experience and therefore outside the reach of any possible theoretical knowledge.

Now the charge of idealism is always the first to arrive, and it must be answered with precision. The position I call transcendental idealism — which I have also called formal idealism, to mark the distinction — holds that space, time, and all phenomena therein are not in themselves things; they are nothing but representations and cannot exist outside the mind.²

But this is emphatically not the claim that there are no external things. I grant by all means that there are bodies without us, things which, though quite unknown to us as to what they are in themselves, we know by the representations which their influence on our sensibility procures us.³ The realist in the transcendental sense — who mistakes these modifications of our sensibility for things subsisting in themselves — commits the deeper error.⁴

And the empirical idealist — who admits the forms of inner sense while doubting the existence of extended bodies in space — commits the opposite confusion, treating inner experience as sufficient proof of the real existence of its object as a thing in itself.⁵ Consider the rainbow. We rightly call it a mere appearance in a sunny shower, and the rain the reality; this is correct enough in the physical, empirical sense.

But if we press further and ask whether the rain itself, considered without reference to the conditions of our sensuous perception, represents an object as a thing in itself — even the circular form of the drops, nay, the space through which they fall, is nothing in itself, but a mere modification of our sensuous intuition, whilst the transcendental object remains for us utterly unknown.⁶

The distinction between rainbow and raindrop is empirical; the distinction between any phenomenon whatsoever and the Ding an sich is transcendental, and at that level, the raindrop is no closer to the thing-in-itself than the rainbow is. The same holds for what were once called the primary qualities — extension, place, space itself. It has long been granted, since Locke at least, that secondary qualities such as heat, color, and taste belong not to things in themselves but to their appearances.⁷

I go further: the primary qualities belong equally to appearance. This is not a larger concession to idealism; it is the completion of a line of thought already begun. The existence of the thing that appears is not thereby destroyed — it is only shown that we cannot possibly know it by the senses as it is in itself.⁸ The faculty of sensibility does not present us with a confused or indistinct cognition of things in themselves; it gives us no knowledge of things in themselves at all.⁹

Abstract in thought our own subjective nature — the forms of sensibility, the conditions of intuition — and the object represented entirely disappears, because it was only this subjective nature that determined the form of the object as a phenomenon. This is the Copernican turn: objects conform to the conditions of our knowledge, not our knowledge to objects considered as they are in themselves.

And yet the Ding an sich is not nothing. It is the limit-concept of theoretical reason, the negative reminder that what we know is appearance and not the noumenal substrate that underlies it. Nature, understood as the existence of things in themselves, could never be cognized either a priori or a posteriori: a priori cognition requires that the object conform to our concepts, but things in themselves do not conform to our understanding; a posteriori cognition through experience can teach us what exists and how it exists, but never that it must necessarily exist so and not otherwise.¹⁰

The thing-in-itself is what remains when every condition of possible experience has been removed — which is precisely why no experience can reach it. The empirical truth of phenomena in space and time is guaranteed beyond any reasonable doubt, and sufficiently distinguished from the illusion of dreams or fancy.¹¹ Objects of experience are real; they are simply not real as things in themselves, but as objects given within the conditions of possible experience.

That there may be inhabitants on the moon, though none has ever observed them, must be admitted — but this means only that we may in the possible progress of experience discover them.¹² What stands in connection with a perception according to the laws of experience is real; what stands outside any possible connection with experience is not an object of knowledge at all, however much reason may be tempted to reach for it.

This is why I call the position critical idealism, and refuse both alternatives: the dreaming idealism that converts actual things into mere representations, and the visionary idealism that converts mere representations into things.¹³ Both confuse the empirical and the transcendental. The Critique was written to hold that distinction firm.

The position in brief

Knowledge is confined to phenomena (appearances under the a priori conditions of human sensibility and understanding) and never reaches things-in-themselves.¹⁴¹⁵¹⁶ Space, time, and the categories (causality among them) are forms the mind contributes, not properties of the noumenal substrate. The Copernican turn: objects conform to the conditions of knowledge, not the reverse.

Synthetic a priori knowledge — mathematics, geometry, the universal laws of nature — is vindicated as articulating the necessary conditions of any possible experience. The thing-in-itself is the limit-concept of theoretical reason: the negative reminder that what we know is appearance, not the noumenal substrate that may underlie it.¹⁷

Sources

the senses never and in no manner enable us to know things in themselves, but only their appearances, which are mere representations of the sensibility
Prolegomena to Any Future Metaphysics, p. 11
time and space, with all phenomena therein, are not in themselves things. They are nothing but representations and cannot exist out of and apart from the mind.
Critique of Pure Reason, p. 107
I grant by all means that there are bodies without us, that is, things which, though quite unknown to us as to what they are in themselves, we yet know by the representations which their influence on our sensibility procures us
Prolegomena to Any Future Metaphysics, p. 11
The realist in the transcendental sense regards these modifications of our sensibility, these mere representations, as things subsisting in themselves.
Critique of Pure Reason, p. 107
The supporters of this theory find no difficulty in admitting the reality of the phenomena of the internal sense in time; nay, they go the length of maintaining that this internal experience is of itself a sufficient proof of the real existence of its object as a thing in itself.
Critique of Pure Reason, p. 107
even their circular form, nay, the space itself through which they fall, is nothing in itself, but both are mere modifications or fundamental dispositions of our sensuous intuition, whilst the transcendental object remains for us utterly unknown.
Critique of Pure Reason, p. 24
it has been generally assumed and granted without detriment to the actual existence of external things, that many of their predicates may be said to belong not to the things in themselves, but to their appearances, and to have no proper existence outside our representation. Heat, color, and taste, for instance, are of this kind.
Prolegomena to Any Future Metaphysics, p. 11
The existence of the thing that appears is thereby not destroyed, as in genuine idealism, but it is only shown, that we cannot possibly know it by the senses as it is in itself.
Prolegomena to Any Future Metaphysics, p. 11
the faculty of sensibility not only does not present us with an indistinct and confused cognition of objects as things in themselves, but, in fact, gives us no knowledge of these at all.
Critique of Pure Reason, p. 24
Experience teaches us what exists and how it exists, but never that it must necessarily exist so and not otherwise. Experience therefore can never teach us the nature of things in themselves.
Prolegomena to Any Future Metaphysics, p. 11
the empirical truth of phenomena in space and time is guaranteed beyond the possibility of doubt, and sufficiently distinguished from the illusion of dreams or fancy
Critique of Pure Reason, p. 107
That there may be inhabitants in the moon, although no one has ever observed them, must certainly be admitted; but this assertion means only, that we may in the possible progress of experience discover them at some future time.
Critique of Pure Reason, p. 107
dreaming idealism, in contradistinction to the former, which may be called 'visionary,' both of which are to be refuted by my transcendental, or, better, critical idealism.
Prolegomena to Any Future Metaphysics, p. 11
Our understanding attains in this way a sort of negative extension. That is to say, it is not limited by, but rather limits, sensibility, by giving the name of noumena to things, not considered as phsenomena, but as things in themselves.
Critique of Pure Reason, p. 188
It is nothing in fact but an objectively valid cognition of appearances and of their succession, so far as the antecedent can be conjoined with the consequent according to the rule of hypothetical judgments. 30. Hence if the pure concepts of the understanding do not refer to objects of experience but to things in themselves (noumena), they have no signification whatever.
Prolegomena to Any Future Metaphysics
The sensuous world is nothing but a chain of appearances connected according to universal laws; it has therefore no subsistence by itself; it is not the thing in itself, and consequently must point to that which contains the basis of this experience, to beings which cannot be cognised merely as phenomena, but as things in themselves.
Prolegomena to Any Future Metaphysics
For to your conceptions of three lines, you could not add any thing new (that is, the figure); which, therefore, must necessarily be found in the object, because the object is given before your cognition, and not by means of it.
Critique of Pure Reason, pp. 39–40

Where the argument is made

Critique of Pure Reason — Transcendental Aesthetic, Transcendental Deduction, Phenomena and Noumena chapter, Refutation of Idealism (added 1787)
Prolegomena (1783)
1772 Letter to Markus Herz

How the argument was received

The first wave of post-Kantians (Reinhold, Maimon, Fichte, Hegel) rejected the thing-in-itself as inconsistent and radicalized into absolute idealism. The twentieth-century analytic reception split between Strawson's attempt to save the analytic of concepts by sacrificing the transcendental aesthetic, Allison's two-aspect reading (phenomena and noumena as one thing under two aspects), and the two-world reading defended by Ameriks. The debate is unresolved; both readings have textual evidence and substantial difficulties.