This is a fascinating topic. Certain schools of philosophy believe that the world as we know it is one of appearances. We do not know it as it really is; we know it only as it is represented in our consciousness. Struck by the unreliability of the senses, Descartes doubted the existence of everything but resolved that, since he was contemplating the world, the act of thinking is the one thing he could be certain of and from that the rest could be deduced. This insight was similar to that of Schopenhauer, who insisted that only our own consciousness is known immediately: everything else is mediated through our consciousness and is therefore dependent on it.
Kant adopted the terms phenomenon and noumenon to distinguish between the thing as it appears to us and the thing as it is in itself. Thus 'phenomenon' applies to the world of appearances, the phenomena that inhabit our consciousness, and 'noumenon' describes the world that 'lies behind' the world of appearances. If we cannot know the world as it is in itself but can know it only through our perceptions, how is it that our perceptions are so uniform we can communicate with each other effectively through a shared view of the world? Kant maintained that humans' capacity to perceive the outside world and thus form representations of it depends on certain 'conditions of sensibility' that exist before the experiences themselves, notably the forms of space, time and causality. These are inbuilt forms of the human mind, and they provide the structure that allows us to receive in an intelligible way the sensory data of the outside world. They are present before experience. As a result, instead of learning the forms of space, time and causality through experience of the world, we in fact impose them on the world. This is the meaning of the often-quoted observation in the Talmud: "We do not see things as they are, but as we are."
Now you may notice that many of these concepts have a similarity with Eastern mysticism. Schopenhauer was the first Western philosopher to appreciate the deep insights of the East, where it is said that the world as we experience it has no real being but is instead 'a ceaseless becoming'. In the Hindu Vedas the idea can be found in the doctrine of Maya, the chimera of the everyday world, 'an unstable and inconstant illusion without substance . . . a veil enveloping human consciousness'.
As with time, so with space: both are forms that inhere not in the world itself but rather in our representations of it. Thus in the Upanishads, space is, in addition to being a property of things, the 'creator' of things, and all things are contained within space as Brahman: "Space, as it is called, is the bringer into being of name and form. That which contains them is brahman, the immortal." All phenomenal forms exist 'inside' the boundless and timeless ground. When the subject brings the form of space to the world, differentiation is created since the spatial coordinates of a thing distinguish it from another thing. In Schopenhauer's words, "only the one and identical essence can manifest itself in all those phenomena." This idea reappeared across the ancient and modern worlds, in the writings of the Platonists and the Sufis and the works of Spinoza and various Christian mystics, before Kant took it up and argued it using the rules of logic rather than just mysticism.
In the Upanishads the idea of oneness from which all else is manifest is explicit in the idea of Brahman. The unifying or subtle essence can be interpreted as the sacred power of the whole universe—that is, the universal 'energy' that is the essence of the noumenon. It is the source of creation yet is uncreated. Thus Brahman is said to be 'self-born'. In Buddhist teaching, which is atheistic, this is captured in the notion of the 'suchness' of the world, the essential quality that infuses all things and reflects the oneness of the whole creation: "When the Ten Thousand things are viewed in their oneness, we return to the Origin and remain where we have always been."
Aldous Huxley summarized the insight of the sages of all traditions by describing a hierarchy of the real:
"The manifold world of our everyday experience is real with a relative reality that is, on its own level, unquestionable; but this relative reality has its being within and because of the absolute Reality, which, on account of the incommensurable otherness of its eternal nature, we can never hope to describe, even though it is possible for us directly to apprehend it."
Apologies for the overly long post.
