In the philosophy of Immanuel Kant, noumenon, thing in itself (German Ding an sich), and transcendental object are nearly synonymous expressions used to refer to the unknowable, indescribable reality that, in some way, lies "behind" the world of observed phenomena. For Kant, we can say nothing at all about things as they are in themselves, because we only know things as they appear to us though our sense perceptions. In time and space, the forms of our intuition are processed by the basic forms (categories) of our understanding. These are the phenomena. Unlike George Berkeley, however, Kant firmly believed that the things we perceive are real; that is, they don’t simply exist in our mind, but knowing what they are in themselves is forever beyond our reach.
It is immediately apparent that Kant’s notion of the noumenon is problematic. It has often been called the crux of his entire system. On the one hand, the existence of "something" behind the phenomenal world is a necessity for him. On the other hand, by denying that we have the capacity to know anything about it, he puts himself in the position of speaking about what cannot be known, which involves a paradox, if not a flat-out contradiction. In fact, Kant himself calls the noumenon a problematic notion, in the sense that it cannot lead to true knowledge. Kant, however, defends its use as a limiting concept (Grenzbegriff); a notion to set clear limits to the ambitions of our mind and remind us that there is something unknowable behind the phenomena.
Kant’s use of the concept of noumenon is closely related to his concept of intellectual intuition. According to Kant, if we had the gift of intellectual intuition, we could perceive the reality of things in themselves as they are, without using the lenses of our sense perceptions, much the same way Plato conceives our knowledge of eternal ideas. Hence, the word “noumenon,” which means intellectual object. For Kant, however, since our knowledge is only possible through our sense perceptions being processed by the categories of our mind, we can neither know the reality behind things in general, nor can we know anything about non-sensible entities postulated by our reason, such as God or the immortal soul. Kant did not recognize a spiritual intuition that would allow us to directly perceive a non-material reality (though he had been fascinated by that possibility, as suggested by Swedenborg, in his earlier years). The closest Kant came to absolute certainty about the transcendent reality was through moral considerations, or faith. This, nevertheless, did not amount to theoretical knowledge for him.
Kant’s contemporary, Jacobi, was the first to notice an apparent contradiction in Kant’s position that the noumenon, in some way, serves as the cause of the phenomenal world, since cause, and even existence, are considered by Kant as mere modes of our thought— they express the way we understand reality, not the way it is—which is unknowable. Further, Kant often refers to noumena (the plural form), though the very notion of individuating items in "the noumenal world" seems problematic, because the notions of number and individuality also belong to the categories of our understanding. In other words, anything that is said about the function of the noumenon automatically denies its unknowable nature and unwittingly relegates it into the world of phenomena.
Kant was aware of the problem, however, and he never directly stated that the noumena are the cause of knowable phenomena. He merely asserted that we must assume that, somehow, a noumenal world exists behind or beyond the world of phenomena, in a non-specified way. In his view, although such an assumption could not be proved, it contained no contradiction. Additionally, our moral or practical reason requests us to assume the existence of a world beyond our senses, the eternity of the soul, and God. This moral certainty, for Kant, takes the place of the theoretical knowledge we cannot achieve.
In sum, Kant’s notion of the noumenon and its relationship to the phenomenal world are not as inconsistent as they have often been depicted to be. Still, the dichotomy between the two realms makes it impossible for Kant to arrive at a fully satisfactory solution.
The notion of the thing in itself or noumenon was promptly rejected as unsatisfactory by Kant’s immediate successors, the German Idealists. Johann Gottlieb Fichte was the first to part ways with his mentor on that point.
Fichte came to the conclusion that Kant’s retention of the thing in itself, unknowable and yet affirmed, was a left-over of dogmatism. Kant had maintained that notion to avoid falling into Berkleyan subjective idealism, the denial that there is a reality outside of our perception. For Fichte, that solution amounted to an illusion. It is not possible for consciousness, he thought, to find a grounding in a supposed real outside world represented by the noumenon.
Instead, for Fichte, the I of Ego had the capacity of perceiving itself directly as a moral agent, bypassing sense perception. The ego does not recognize itself as it would recognize an object, but as an immediate awareness of its own moral activity that cannot be further demonstrated. In that sense, Fichte believed Ego has intellectual intuition. Based on Kant’s own moral certainty of the categorical imperative, Fichte thus moved the pursuit of the thing in itself from the realm of theoretical knowledge to that of inner certainty. The thing in itself had become one with the Ego. Thus, with Fichte, a new form of subjective idealism had appeared.
For Hegel, things-in-themselves, as understood by Kant, are "mere abstractions, void of truth and content" (Science of Logic). They are nothing more than the empty self-identity of the ego made into an object. Linking Fichte’s Ego with Friedrich Wilhelm Joseph von Schelling’s notion of the Absolute, Hegel developed a system where the thing in itself became the self-realization of Absolute Thought.
Unlike the German idealists, Arthur Schopenhauer identified himself as a direct heir to Immanuel Kant and he took over Kant’s notion of noumenon, identifying it with the Will. Like Fichte, Schopenhauer felt that Kant was mistaken in positing the noumenon or thing in itself as the ground of the phenomenal world.
Schopenhauer noted that the opposition between noumena and phenomena had a long history dating back to Greek philosophy and continuing with the opposition between realism and nominalism in Scholastic philosophy.
The phenomenal world, which Schopenhauer calls the World as Representation, in his eyes was nothing but the representation of our mind, as was the case for Berkley, whom he greatly admired for this insight. The noumenon, Schopenhauer, felt, had to be sought on an entirely different level, the World as Will. Though he thus identified the unknowable noumenon as an entity to which we can directly relate, Schopenhauer ended up with a problem similar to that of Kant. It became difficult for him to explain the relationship between the noumenal world of the Will and the phenomenal world of Representation.
"Noumenon" is a borrowed Greek word meaning something 'contemplated or perceived'. It is the neuter of present passive particle of noein (to think, to conceive), which originates from nous (mind). It is linguistically unrelated to "numinous," a term coined by Rudolf Otto and based on the Latin numen (deity).
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