Summary: Critique of Pure Reason (page 11)

Space is therefore not an empirical concept derived from experience. On the contrary, it is one of the a priori forms of our sensibility, through which experience itself becomes possible. This is why space is necessary—we inevitably employ it in our representations. Only what is a priori, unlike the empirical, can give rise to necessity.

Space is not a pure concept but a form of pure intuition, because it is infinite, whereas we cannot conceive of any concept that contains within itself an infinite multitude of representations ¹.

Geometry offers further proof of the a priori character of space. Geometry is a science that synthetically and yet a priori determines the properties of space. The a priori—and therefore necessary—character of its deductions stems from the fact that space is a pure form of intuition. Were it merely an empirical intuition, geometrical deductions would not be necessary.

Kant demonstrates the same for time, showing that it too is a pure form of sensible intuition:

With regard to phenomena in general, one cannot remove time itself, although one can very well think of phenomena as removed from time.

Time is thus a form of pure intuition.

This is a crucial point: objects are spatio-temporal because space and time are the forms of sensibility. To attempt to intuit objects outside of space and time would be like trying to see without eyes.

To return to our earlier metaphor of pink-tinted glasses: we necessarily see the world in pink because the pink is part of the lenses themselves. Trying to stop seeing in pink—to see the "true" colours, unaltered by this filter—would be like trying to remove the glasses, which is something we cannot do.

In the same way, we perceive objects in space and time because space and time are integral to our cognitive faculties. To attempt to escape them would be like trying to know without using our faculty of knowledge.

What are the consequences of this theory?

They are far-reaching.

First Consequence: Space and Time

First of all, space and time do not exist in themselves. Space and time do not belong to things as they are in themselves but to the mind. They are forms of sensibility that we mistakenly take for real relations between things. In reality, they are merely conditions of our mind that make experience of these things possible:

Space does not represent any property of things in themselves, nor the relations these things bear to one another. Rather, space is nothing but the form of all appearances of external sense—that is, the subjective condition under which alone an external intuition is possible for us.

Thus, we can only speak of the space of extended beings, and so on, from a human perspective. If we abstract from the subjective condition under which alone we can receive external intuitions—namely, our capacity to be affected by objects—then the representation of space means absolutely nothing. The same holds for time.

Second Consequence: The Impossibility of Knowing the Thing-in-Itself

Let us call the thing-in-itself the thing as it truly is, unaltered by our cognitive faculties. The thing-in-itself represents, in a sense, the absolute truth we aim at when seeking to know things as they really are.

With Kant, the idea is definitively established that we cannot know things-in-themselves. We can only know phenomena—that is, things as they have been modified to conform to our cognitive faculties:

All our intuition is nothing but the representation of appearance; the things that we intuit are not in themselves as we intuit them.

We do not see things as they truly are. In experience, we do not perceive things in their absolute reality—as things-in-themselves—but rather a kind of mixture of things and mind, of things-in-themselves and the forms of sensibility.

Failing to make this distinction, we mistakenly take what belongs to the mind—space and time—as something that belongs to the real world.

¹ Our translation. The references for the quotations are available in the book Kant: A Close Reading