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, and this must be the starting point for any philosophizing. 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.
But can reality be nothing more than our representations of it? Is it possible that nothing exists as such? Kant argued that if the world is one of appearances there must be something that appears, 'an object independent of sensibility'. He referred to this as the 'thing considered as it is in itself', or the 'thing-in-itself' for short. The thing-in-itself must be neither object nor subject. And if it transcends the distinction between subject and object it must transcend consciousness and the knowledge that attends consciousness.
If this is so, in addition to being an object of consciousness, everything is also a thing-in-itself. This 'existence in itself' stands outside the subject – object relationship and has a metaphysical existence that has its being in an unknowable world beyond consciousness; it is a 'metaphysical being'. We can know nothing about it because all we know are appearances, and those appearances are structured by our given forms of understanding.
At this stage of the argument it seems reasonable to imagine that the world exists twice—once as an 'objective fact' independent of us and once in our representations as a set of images projected onto some sort of screen in our head. As the argument develops, though, it will become apparent that there is just one world but there are two distinct ways of apprehending it.
The main argument for the existence of the thing-in-itself is that without it we are left with a view of the world as nothing more than the illusion created by human consciousness. Although we know everything we know through our consciousness—including our senses and our cognition—this is not to say there is nothing we cannot know. Our awareness has limits. It is this that allows for the possibility of things existing beyond their appearances, as the thing-in-itself.We can think about the world 'beyond' the world of appearances, but that does not mean we know it. To use a theistic analogy, the idea of God can never be God.
The approach of transcendental idealism—which considers the problem of reality not from the starting point of object or subject but through the inseparability of the two—differs from other philosophical approaches, including the commonsense realist one. Most philosophical approaches, at least in the Western tradition, recognize only the subject or the object. In particular, realists (or 'materialists') recognized only the object, and the idealists who preceded Kant recognised only the subject. Realists take matter as having absolute existence, independent of consciousness, but they are then confronted with the problem of perception: if I see an object as green but my colour-blind friend sees it as red we are compelled to admit we cannot do without the perceiving subject. As Schopenhauer wrote, "For 'No object without subject' is the principle that renders all materialism for ever impossible".
Realism, which coincides with the working assumption of everyday life, is therefore based on nothing more than a supposition—the absolute existence of the world beyond our senses—since we can never test the supposition by stepping outside ourselves to see whether it is true. Such a world is not conceivable. Thus, despite its intuitive obviousness, realism is "the philosophy of the subject who forgets to take account of himself. "
Mysticism has been called 'a doctrine of epistemological modesty': it recognises the limits of our knowledge. This might seem odd at first, but on reflection we see that realism imagines there is a world outside us which is then mirrored by a world within us that represents the 'objective' world. In this view we imagine, if only by implication, an all-seeing God above who observes his creation, a God who exists independently of our consciousness. This is implicitly a theocentric model, and Kant's revolution was to make our understanding of the world human-centered instead of God-centred—perhaps the crucial philosophical insight of the European Enlightenment. The world as we see it is as we see it, rather than as it really is; it is conditioned by our perception; it is a world of appearances. This is the meaning of the often-quoted observation in the Talmud: "We do not see things as they are, but as we are."
Kant did not turn man into God but took away from man the god-like insight the realists had unwittingly assigned to him. This is why Kant's idealism is modest and why Kant himself was suspected of holding heretical views. Kant's intervention followed a long and intense debate in the Catholic church over the nature of reality, the theologians being divided between nominalists, realists and conceptualists. As the Enlightenment dawned the consensus converged on what was known as 'moderate realism' (especially as formulated by Thomas Aquinas), which allowed the church's teachings to coexist with the new science. Kant's assertion of a form of conceptualism laid the groundwork for the emergence of humanism. In particular, the anthropocentric model gave humans responsibility for their moral laws and their destiny, rather than these things being handed down by God, with the clerics acting as middlemen.
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. For Kant, the difficulty lay in establishing the need for the existence of something about which we can know nothing. Positing the existence of the noumenon was pivotal because if the world were nothing but appearances it would be indistinguishable from a dream. But if there is another side to the world we can know nothing about it, since anything we do know takes the form of a representation, a phenomenon.
Appearances however must be appearances of something. Then the noumenon is a thing 'so far as it is not an object of our sensible intuition'. But there could be a positive reason for establishing the existence of the noumenon. At one point Kant entertained the idea that there might be a special kind of awareness, beyond our ordinary consciousness, that allows us special knowledge of the noumenon. He suggested "a peculiar mode of intuition . . . the very possibility of which we have no notion" that would allow us to understand the noumenon
positively, as "an object of a non-sensible intuition". But he then rejected such a notion and reverted to negative arguments for the noumenon.
Although declaring that "we are not entitled to maintain that sensibility is the only possible mode of intuition", Kant became known, adopted a position of philosophical humility, shying away from this postulated form of non-sensible intuition because such a notion "lies absolutely beyond what we could know". He conceded the possibility of a form of intuition beyond everyday senses and cognition but argued that since our faculties can know nothing of this it is legitimate to motivate the existence of the noumenon only in the negative sense. Kant's 'distinctive concern was to vindicate the authority of reason' after centuries of superstition and ecclesiastical dogma. He was determined to stay true to the Enlightenment value of stringent rationality and resisted the temptation to resort to notions that seemed dangerously close to theology. So, despite professing to be a Christian throughout his life, he used the existence of the noumenon to structure the void left by the boundedness of the world of appearances—as if to prevent theists from filling it with God. Thus, in the end he insisted that the division between phenomenon and noumenon can be supported only in the negative sense. The willingness to argue for the existence of a form of 'nonsensible intuition' was the decisive break from Kant that Schopenhauer made.
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 a priori—that is, 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 a mind-bending idea. Putting causality aside for the moment, in this view space and time are the two pure forms of sensory intuition. Space is the property of the mind by which we represent objects outside ourselves, whereas we represent everything that relates to our inward mental or sensory consideration in terms of time. Space and time thus provide the pure forms by which we apprehend the world; they do not belong to things in themselves. In the case of space, we do not learn by experience to understand and use the concept because it already structures our capacity to perceive the world. It is 'the foundation of all external intuitions' or appearances. Indeed, the priori form of space makes all external appearances possible. Geometry is the science of space that developed wholly independently of any real object and does no more than apply intelligence to the form of space. If we imagine an object from which we progressively strip away all features—colour, hardness, weight, and so on—we would still be left with the space it occupies, 'and this it is utterly impossible to annihilate in thought'.
So, rather than being properties inherent in the 'object' (the material world outside us), these priori forms are brought by the subject to the world and used to structure representations of it. Indeed, these forms are essential to allow communication between subject and object—that is, the opportunity for the subject to structure the world in a useful way. This is a radical and counter-intuitive position since Kant is arguing that, instead of assuming that 'our knowledge must conform to the objects' as they are actually given outside of us, 'the objects must conform to our knowledge'. Although this might seem a monstrous idea, because humans are seen to 'create' the world, the opposite is the case. Instead of humans being able to capture reality as it is, we are captives of our representations of the world and cannot know it as it really is.
According to Kant, humans provide the rules by which we understand the world. Kant called these rules we bring to the world the 'legislation for nature'. They allow us to order our perceptions. And they make possible pure understanding—cognition that depends on concepts alone, such as pure mathematics. The rules we bring to the outside world are common to all of us and therefore give our understanding objectivity. Realists, on the other hand, do not recognize these rules as priori conditions; instead, they attribute them to the outside world as inherent qualities of it that each human must learn anew through experience.
The properties of the world as we perceive it do not belong inherently to the world but are 'imposed' on the world by our perception of it. This does not mean the only reality is the world of appearances; rather, it means that, to the extent that we perceive and think, we do so only in terms of appearances. There is a reality 'behind' appearances, or at least it is necessary to posit such a world because appearances presuppose things that appear.
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 has been understood for millennia that the world as we experience it has no real being but is instead 'a ceaseless becoming'. In the Hindu Vedas—the oldest books in the world—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'. These ideas, Schopenhauer noted, were produced not from philosophising by the sages but from 'the direct utterance of their 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 (a later distillation of the Vedas), 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 the Kantian view, the phenomenon is characterised by multiplicity but the noumenon is undifferentiated; 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 direct realisation.
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 summarised 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."
