Noumenon

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The noumenon is the object considered as it is in itself, apart from the way it appears to a perceiving mind. The word entered modern philosophy through Immanuel Kant, who used it as the formal counterpart to the phenomenon — the thing as it is given in experience — and who often spoke instead of the Ding an sich, the thing-in-itself. Behind every appearance, Kant held, lies something that appears; what that something is, in its own nature, the human intellect is not equipped to know. The distinction is the spine of the Critique of Pure Reason (1781, with a heavily revised second edition in 1787): a single boundary drawn through the whole field of the knowable, with everything science can reach on one side of it and, on the other, the bare thought of what lies past the reach.

Immanuel Kant, who introduced the noumenon as the formal counterpart of the phenomenon in the Critique of Pure Reason (1781). — Unidentified painter, via Wikimedia Commons (public domain)

The term is older than Kant, and his use of it bent it sharply. In Greek, nooumenon — the passive participle of noein, to think or apprehend — meant that which is grasped by nous, the intellect, as opposed to aisthēton, what is taken in by the senses. This is Platonic vocabulary, and in it the relation between the two orders runs the other way from Kant’s. For Plato the intelligible was the more real: the changeless objects of thought stood behind the flux of sensation as its ground and pattern, and the soul that turned from the shadows of the cave toward them was turning toward being itself. The whole architecture of later Platonism and Neoplatonism is built on that confidence — a hierarchy of intelligible reality rising through Nous toward the One, each level more truly being than the sensible world that depends on it. To call something noumenal, in that idiom, was to locate it higher, nearer the source, more available to the mind precisely because the mind is the faculty fitted to it.

Plato (left), pointing upward toward the intelligible realm, in Raphael’s School of Athens*; for Plato the objects of thought were the more real, the ground behind the flux of sensation. — Raphael, c. 1509-1511, via Wikimedia Commons (public domain)*

Kant inherited the thought-versus-sense contrast and kept it, but he reversed the optimism that came with it. For the Platonist the intellect’s reach was the cure; for Kant it was the disease to be diagnosed. Space and time, he argued, are not features of things in their own right but forms the mind brings to whatever it receives — the conditions under which anything can be given to sense at all. The categories of the understanding (substance, causation, and the rest) are likewise the mind’s own forms, the rules without which the manifold of sensation could never be combined into the experience of an object. These forms make knowledge possible, and in the very same stroke they fence it in. We know objects only as they are shaped by the apparatus that lets us know anything; the object as it stands free of that apparatus falls outside what can be cognized. The noumenon is the name of that outside.

So the concept arrives mainly as a limit, and Kant was careful about what kind of limit. He drew a distinction, sharpest in the chapter of the first Critique on the ground of the division of objects into phenomena and noumena, between two senses of the term. The negative noumenon is the thing merely insofar as it is not an object of our sensible intuition — a placeholder marking where our way of knowing leaves off, a boundary stone and nothing filled in beyond it. The positive noumenon would be an object of a non-sensible, purely intellectual intuition: a mind that could reach its objects directly, without being given them through the senses, and so could know things as they are in themselves. Such an intuition Kant ascribed, if to anything, to a divine intellect; he firmly denied that human beings possess it. The positive noumenon is therefore a coherent thought we can frame but never use. We are entitled to the negative noumenon as a boundary concept — a Grenzbegriff, in his term — and to no more. The intelligible world the Platonist meant to enter, Kant retains only as the shape of the wall.

This is the point at which his readers have worried ever since. If the thing-in-itself is unknowable, how can it be said to exist at all, or to affect us, or to cause the appearances we have — when existence, reality, and causation are precisely among the categories Kant restricts to the phenomenal field? To apply causation to the noumenon, the objection runs, is to use the very tool whose use beyond experience the Critique forbids; to deny that the noumenon affects us is to cut the appearances loose from anything that appears. The horns were stated with unusual force by Friedrich Heinrich Jacobi, who put it that without the presupposition of the thing-in-itself he could not enter the critical system, but with it he could not remain inside it. The presupposition is needed to get the system started — something must be there to be appeared-to — and forbidden once it is running. Jacobi meant this as a charge that Kant had quietly helped himself to knowledge of the very thing he had declared unknowable.

The German Idealists who followed treated that instability not as a flaw to be patched but as the loose stone on which the whole edifice turned, and built their philosophies by trying to remove it. Fichte concluded that a thing-in-itself lying wholly beyond the knowing subject was an incoherent residue and dissolved it into the activity of the self that posits its own world. Hegel pressed further, dismissing the unknowable beyond as an empty abstraction — the thinnest of all thoughts, a something of which nothing whatever can be said — and held that the supposed limit, once thought through, collapses, since to mark a boundary is already in some manner to have crossed it in thought. For both, the noumenon was the symptom of a philosophy that had stopped one step short. Where Kant kept thought and its object permanently sundered, idealism after him moved to close the gap and make the real, at bottom, the rational.

G. W. F. Hegel, who dismissed the unknowable thing-in-itself as an empty abstraction and held that to mark a boundary is already in some manner to have crossed it in thought. — Julius Ludwig Sebbers, via Wikimedia Commons (public domain)

Schopenhauer took the opposite turn from the same starting point. He kept the phenomenon-noumenon distinction and honored it as Kant’s great achievement, but denied that the thing-in-itself must stay sealed off. There is, he argued, one case in which we are not merely outside an appearance looking on, but are the thing that appears: our own body, given to us from without as an object among objects and from within as will. That inner access is not another inference across the causal gap — which would only fall back under Jacobi’s objection — but a different mode of acquaintance altogether, the world known from the inside. Generalizing it, Schopenhauer named the thing-in-itself will: a single, blind, groundless striving underlying all appearance, lying outside space, time, and plurality, of which the entire phenomenal world is the objectification. The unknowable had been given a name and a character — though at the cost, his critics note, of the very humility about the noumenon that had been Kant’s point.

Arthur Schopenhauer, who kept Kant’s distinction but named the thing-in-itself will*, reached through the body known from within. — Ludwig Sigismund Ruhl, 1815, via Wikimedia Commons (public domain)*

What Kant himself meant by the distinction remains contested, and the dispute is not merely historical. On one reading — the two-world or two-object view — phenomena and noumena are two distinct sets of entities: the appearances we know and, behind or beneath them, the wholly other things-in-themselves we do not. On the rival two-aspect reading, advanced most influentially in Henry Allison’s Kant’s Transcendental Idealism (1983; revised 2004), there is one set of objects considered in two ways — as they appear under the conditions of human sensibility, and as they are when those conditions are abstracted from. The contrast is then methodological rather than ontological: a single world thought under two standpoints, the human and one no human can occupy, rather than two populations of things. Much turns on which is right. The two-world picture makes the causation objection bite hardest; the two-aspect picture promises to defuse it, at the price, its critics say, of softening the thing-in-itself into a manner of speaking. The argument proceeds by close reading of notoriously compressed passages, and the texts do not force a verdict.

The textual ground of all this is narrow and much-traversed. The primary locus is the Critique of Pure Reason — the Transcendental Aesthetic on space and time, and above all the chapter on the ground of the distinction of all objects into phenomena and noumena, where the negative and positive senses are set out and the positive noumenon assigned to an intellectual intuition humans lack. Of the modern scholarship the standard pillar of the English-language debate is Henry Allison’s defense of the two-aspect reading, and the literature is well mapped in the Stanford Encyclopedia of Philosophy entry on Kant’s transcendental idealism, which lays out the two-world and two-aspect camps and the affection problem at issue between them. Jacobi’s framing of the dilemma — the presupposition needed to enter the system and fatal to staying in it — is documented in the Stanford Encyclopedia entry on Friedrich Heinrich Jacobi, and the recovery of an inner route to the thing-in-itself is traced in the entry on Arthur Schopenhauer, where the body known from within becomes the single window onto the will. Recent work keeps reopening even the basic question of how strictly Kant meant to identify the noumenon with the thing-in-itself, the two terms carrying subtly different loads across the Critique.

The idea reaches well beyond academic philosophy. Wherever a tradition holds that the real lies behind or beneath what the senses report — the veil of appearance in Indian thought, the hidden God of apophatic theology and the mystics, the conviction running through several currents that ordinary perception is a kind of sleep — the structure rhymes with Kant’s, and the rhyme has tempted many to hear in the noumenon a philosophical license for gnosis, or to read the acosmist who calls the finite world unreal as a cousin of the man who called it mere appearance. The resemblance is worth marking and easy to overstate. The mystic’s veil hides a presence to be reached; Kant’s boundary hides nothing that could be reached at all, and the apophatic theologian’s unknowing is a way toward God, not a fence forbidding the road. Kant was drawing a boundary around knowledge, not promising a way across it; the mystic claims to cross. What they share is only the conviction that the world as given is not the world entire.

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