Quora Answer: Immanuel Kant: If the concept of space/time is an a priori intuition, then—just as it’s impossible to know noumena—wouldn’t it be impossible to know whether space/time exists independently of us?
There is four states according to Kant:
Synthetic A priori
Analytic A priori
Synthetic A posteriori
Analytic A posteriori
Both Space and Time are Synthetic A priori notions
And also the Categories in Space and the Schema in Time are Synthetic A priori notions
Synthetic A priori is something we project prior to experience that is necessary to make experience possible.
Analytic A priori is any sub-notions that can be deduced from the categories or space and time. So for instance for Kant geometry is deduced from space and arithmetic deduced from time.
The thing-in-itself is beyond these projections. We really don’t know it or how it interacts with the projections to fill the world with sensations that become perceptions. But what we do is intuit things based on the presented manifolds of the A priori syntheses we perceive (with our senses) and apperceive (with cognition). The combination of the two are experience. The thing-in-itself is genuinely transcendent in the sense of not being available in any way and not being known in any way. But because of that it is really just a phantom of our imagination anyway, in fact it does not exist, and is only there as a part of the philosophical architecture to make sure that we know that everything we experience is phenomenological. The thing-in-itself is what Husserl would call a bracket, it is there so we know that what ever we posit beyond experience is out of bounds for metaphysical speculation. Prior metaphysics to Kant had all kinds of theories about the thing-in-itself, but for Kant this kind of dogmatic speculation is out of bounds and a bad use of reason.
That means that “objective” space and time beyond experience are themselves noumena. But experienced space and time as phenomena are synthetic a priori projections only. But they are different from other projections in as much as they are singular, in other words there is only one space and only one time that is projected everywhere by everybody. And in fact for Kant that is the truly “objective” space and time. Today we would say spacetime, but that they were both the same singular was unknown to Kant.
There is a direct line from Kant to Husserl. Husserl in his phenomenology attempts to get at what is happening in consciousness within the Kantian Transcendental Framework. Transcendental means necessary for the possibility of experience like Space, Time and the Categories. Transcendent means beyond experience like the noumena. Kant is a Transcendental Idealist and believes that is the only route to Transcendental Realism. Because the transcendent noumena is never known, and can never be known, our only access to reality is via idealism. Idealism for Kant is the only possible realism. When you bracket the thing-in-it-self, or noumena, completely then there is nothing else but what appears in experience, and that is real. Thus what ever is real is exactly what appears in experience. Abstractly this appears quizzical because we can continue to talk about noumena, and when we do we are really just making out of bound metaphysical claims which is a sickness of reason for Kant.
But Husserl in his thought makes a fundamental contribution when he gives up bracketing, that has all kinds of problems like the problem of intersubjectivity, and instead talks about things as appearing on the horizon of the world. This solves all the problems of the noumena, and all the problems of bracketing because it says that every thing in experience is on the greatest possible horizon the horizon of everything in the world. And things have reality to the extent that they can be explored. Real things can be explored almost infinitely and that is what makes them real. Imaginary things have some limit to their exploitability on the horzon and that is what makes them less than real. The horizon of the world is intersubjectively constituted and not rooted in the individual alone and thus the problems of intersubjectivity as in Cartesian Meditations vanish. So if we update Kant with the insights of the later Husserl then we see that what seem to be problems of Realism that linger in Transcendental Idealism are artifacts of the way Kant talked about the noumena as a kind of bracketing mechanism in his overall philosophical system which is basically phenomenological but Kant had not discovered phenomenology as a perspective yet. We had to wait for Hegel and also Pierce for that. Husserl refined Phenomenology and got rid of the excesses of Hegel as Peirce was also trying to do. That is to say Husserl and Peirce were trying to create a Kantian Phenomenology. But Husserl is the one who had the real insight as to how to get rid of this problem that lingered of realism verses idealism, by realizing that if there is a horizon of the world in which everything appears then it is the almost infinite exploitability of some phenomena that makes it seem real, and we can gauge all other experiences of phenomena against that infinite exploitability to judge their reality. And because it is an intersubjectively projected horizon we can do science on those objects we deem real. It took a long time to refine the Kantian Idealism in order to solve the problems of understanding transcendental realism and how Idealism is our only access to what is real without positing the brackets of the noumena.
Heidegger, of course, is assuming all this. His Dasein as being-in-the-world is merely that part of us that does the projection. What Heidegger is trying to do with Dasein is get beyond the Subject/Object dichotomy that still plagues Husserlian Phenomenology. And in Heidegger’s dasein we see the basic and underlying paradox of the whole Kantian and Husserlian Idealism which is that Dasein projects the world and then is an entity within the world it projects. But it is not yet a subject nor an object, that is a Present-at-hand reification that happens later. Rather Dasein exists in a mode of the Ready-to-hand where it exercises circumspective concern in a Process Being mode over the technological infrastructure that underlies all experience that is presented as present-at-hand, i.e. in a Pure Being mode. These two modalities of Being (present-at-hand and ready-to-hand) are equi-primordial and in fact always present in each moment together, whether we emphasize one or the other. But they are ways of pointing at things and grasping things that appear on the World Horizon of our being-in-the-world as explained by Merleau-Ponty in The Phenomenology of Perception.
Really it is necessary to have the perspective of the history of Philosophy in order to understand any one particular contribution. For instance we really need to understand Descartes, Spinoza, Leibniz and Locke and Hume in order to situate the contribution of Kant in his attempt to ground the physical science of Newton. That grounding still stands, no one has gotten beyond the transcendental framework set up by Kant. He posited fundamental limits to what reason can know beyond experience, basically positing that we cannot know anything beyond experience, but we can see in experience certain necessary elements that make experience possible and for us those are transcendental, but what is actually beyond experience, i.e. the bracket of the noumena, we cannot know.
Schopenhauer gives an interesting twist to this, by saying that we as entities must ourselves have a noumena within us, and he identifies that with Will and he says that we have special access to that Wille via music. This is a very interesting case which Nietzsche takes up and connects to evolution via his idea of the Will to Power. Schopenhauer gave a basis for romantic reactions against Kant with his concept of Will being the thing-in-itself within us. His term Wille is wider in meaning than our term Will. This basically gives rise to the concept of the Unconscious via Nietzsche. And as we know via Freud and Jung who explored the territory that Nietzsche opened up (but denied or repressed Nietzsche in the process) the unconscious has become one of the greatest modern concepts to be reckoned with. And if we were to say that there is any road to partial intimations of glimmers of the noumena it would be through the unconscious in ourselves, our noumena may not be as closed off from our experience as the noumena of other things like objective scientific objects. Thus the story does not yet have an end. Continental Philosophy has been exploring via Lacan and those that take him seriously like Deleuze the implications of the idea that the unconscious is in fact a noumena whose effects we see directly in its warpages of consciousness even though we have no direct access to it. And of course this brings up the question of access to other noumena of other things than ourselves. Jung develops the idea of the Psychoid as a way of thinking about this in psychological terms. So although the question seemed to be closed with regard to the reality beyond experience of objective things it is not completely closed with regard to the reality of what underlies subjective things