Critique of Pure Reason

There are two pure forms of intuition: space and time. Space is the outer sense of intuition whereas inner experience is constituted by the passage of time. Kant writes that “space is not an empirical concept that has been abstracted from outer experience” (77). On the contrary, space is a necessary condition for the possibility of outer experience. As such, it cannot be situated in outer experience but must be brought to it. In other words, it is a subjective condition, not an objective condition, of outer experience. Despite this, the intuition of space is still a universally necessary (transcendental) subjective condition. It is required for all human cognition in every possible circumstance. Unlike taste and color, which are contingent and differ from individual to individual, space is one and the same for all. There is only one space, though it can be arbitrarily divided into parts. We present it as “an infinite given magnitude” (79). Kant seeks to provide an “transcendental exposition” of space by which he hopes to show how synthetic a priori propositions can follow based on the concept. Kant uses geometry as an example. Geometry is made possible because of this explication of space as a transcendental condition for outer experience.

Mathematical/geometrical propositions are deductively valid, meaning that they are always true in all cases. This is not the result of empirical observation but is necessary a priori. The a priori determination of space as a transcendental condition for outer sense, which requires, for instance, three dimensionality, provides an explanation that makes the possibility of geometry understandable. So, while space is empirically real, in that it pertains to all outer experiences, it is, more fundamentally, “transcendentally ideal” (82). The external objects we encounter in space are only appearances, or presentations, that are the result of our cognitive capacity of sensibility. The thing behind the appearance, or “thing in itself,” can never directly be the object of experience (84). 

The metaphysical exposition of time parallels the same exposition of space. However, whereas the concept of space refers to intuition of outer experience, the concept of time refers to intuition of inner experience, or inner sense. Like space, time is not an empirical concept. It is not grounded in experience but is, rather, the ground of experience. In other words, “time is a necessary presentation that underlies all intuitions” (86). Apodeictic principles, those that are beyond dispute, are derivable from time when understood as an “inner sense.” The principle that time proceeds sequentially (not simultaneously) is one such principle. It cannot be derived from experience but must be the result of the nature of time itself. Time is present to consciousness as unlimited. It is unlimited sequentiality through which all determinate portions of time are parceled.

Just as the transcendental exposition of the concept of space elucidated the possibility of geometry, the transcendental exposition of time “explains the possibility of all that synthetic a priori cognition which is set forth by the–quite fertile–general theory of motion” (87). Also, analogously to the concept of space, time is not merely empirically real but is (more fundamentally) transcendentally ideal. While it holds with objective certainty for all possible experience (making it empirically real), at base it is a formal, subjective condition of the human mind. Kant writes that we cannot claim that time has any absolute reality. In abstraction from our capacity for sensibility, Kant says, “time is nothing” (90). Time is even more general to human experience than space insofar as it is the “inner intuition” of sensibility, and outer intuition must pass through inner intuition. In short, all experience is unavoidably placed in time. Time also applies to outer sense, or intuition constituted by space.

According to Kant, the transcendental aesthetic is a completed science when it understands space and time. There cannot be any more elements because “all other concepts belonging to sensibility presuppose something empirical” (93). Transcendental conditions are necessarily a priori metaphysical conditions and therefore nonempirical.

Kant completes his engagement with the transcendental aesthetic. He concludes that we cannot know objects as they are independent of our perception. We cannot abstract from our sensible intuitions of space and time. “Even if,” Kant writes, “we were able to see through that appearance and to its very bottom” we would still not have knowledge of the object as it is itself, apart from our intuition of it (96). This object beyond appearance is known as the transcendental object. It must exist for the possibility of experience, but it is not ever presented within that experience. The pure intuitions of space and time constitute the formal parameters of sensibility through which we receive the objects of experience. Kant writes “our kind of intuition is called sensible because it is not original” (103). Therefore, the activity of our spatial intuition is determined through the existence of objects external to us. Even if we can only be aware of these objects through the way they appear to us, we know that these appearances are something external to us. Appearances are not illusions. The objects that are sensed, or received, are external to the mind via the outer sense of space. Inner sense, or time, is internal to the mind, but nevertheless still only presents objects as appearance. Even the appearance of the self, the I (the subject), is present merely as an appearance: “The consciousness of oneself (apperception) is the simple presentation of the I” (100). Our intuition of ourselves is merely sensible and therefore no different from any other intuition. Having acquired knowledge of the nature of space and time, Kant writes that he has made headway on answering the question regarding the possibility of synthetic a priori propositions, which is the fundamental problem to be answered for transcendental philosophy.