The Transcendental Aesthetic is the first major section of Kant’s monumental Critique of Pure Reason. Found in this section are his arguments for the transcendental ideality of space and time. Against the view that space is a property of things, or a relation between them, Kant argues that space and time are merely the necessary forms of our representation.
To make this case, his argument moves in two directions. Firstly, the previous views of space lead to impossibilities, which he illustrates one by one. To take one example, against the empiricists, Kant argues that we could never derive an idea of space from experiences of contiguity, because such experiences already presuppose contiguity. That is, we need to first understand space in a general way to understand that objects are ‘next to’ or ‘behind’ one another, or even to understand they are ‘outside’ of us at all. We can’t derive an idea of space from experiences of spatial relationships, because we need the idea to grasp the relationships, it is their condition of possibility.
Secondly, he presents a positive argument. That space is an ideal form of experience is the best explanation for how it is that geometry can, on the one hand, make interesting discoveries and expand, and, on the other hand, prove its point with absolute, mathematical certainty. If space were something discovered through our experience, our knowledge of geometrical principles would be resigned to being either definitional (thus not interesting or capable of expansion) or merely probabilistic: just likely rather than necessary.
It is not hyperbole to claim that, for better or worse, the arguments contained in The Transcendental Aesthetic are some of the most important in the history of philosophy. They mark a radical solution to the problems of Early-Modern philosophy, and the creation of new possibilities for philosophy.