Space and time do not have an existence "outside" of us, but are the "subjective" forms of our sensibility and hence the necessary a priori conditions under which the objects we encounter in our experience can appear to us at all.
[8]: 57 If we try to keep within the framework of what can be proved by the Kantian argument, we can say that it is possible to demonstrate the empirical reality of space and time, that is to say, the objective validity of all spatial and temporal properties in mathematics and physics.
This condition of experience is part of what it means for a human to cognize an object, to perceive and understand it as something both spatial and temporal: "By transcendental idealism I mean the doctrine that appearances are to be regarded as being, one and all, representations only, not things in themselves, and that time and space are therefore only sensible forms of our intuition..."[10] Kant argues for these several claims in the section of the Critique of Pure Reason entitled the "Transcendental Aesthetic".
Such insight is bound up with the understanding why such knowledge is this and has this power, namely because it constitutes the form of our intellect, and thus in consequence of its subjective origin ... Transcendental is the philosophy that makes us aware of the fact that the first and essential laws of this world that are presented to us are rooted in our brain and are therefore known a priori.
From this it follows also that the objective world as we know it does not belong to the true being of things-in-themselves, but is its mere phenomenon, conditioned by those very forms that lie a priori in the human intellect (i.e., the brain); hence the world cannot contain anything but phenomena.In The Bounds of Sense, P. F. Strawson suggests a reading of Kant's first Critique that, once accepted, forces rejection of most of the original arguments, including transcendental idealism.
[13]: 99–101 They are tagged as "phenomena" to remind the reader that humans confuse these derivative appearances with whatever may be the forever unavailable "things in themselves" behind our perceptions.
In Kant's Transcendental Idealism, Henry E. Allison proposes a new reading that opposes, and provides a meaningful alternative to, Strawson's interpretation.
[14] Allison argues that Strawson and others misrepresent Kant by emphasising what has become known as the two-worlds reading (a view developed by Paul Guyer).
On Allison's reading, Kant's view is better characterized as a two-aspect theory, where noumena and phenomena refer to complementary ways of considering an object.
Allison's two-aspect interpretation also serves as an at least partially successful defense of transcendental idealism, particularly within anglophone analytic philosophy.
[16] Opposing Kantian transcendental idealism is the doctrine of naïve realism, that is, the proposition that the world is knowable as it really is, without any consideration of the knower's manner of knowing.
Naïve or direct realism claims, contrary to transcendental idealism, that perceived objects exist in the way that they appear, in and of themselves, independent of a knowing spectator's mind.
[citation needed] Kant referred to this view as "transcendental realism," which he defined as purporting the existence of objects in space and time independent from our sensibility.