The Third Part deals with the question, "How is metaphysics in general possible?" We have seen how both mathematics and pure natural science are possible, by appealing to our pure intuitions of time and space and the concepts of our faculty of understanding. We use our pure intuitions and our faculty of understanding to make sense of experience, but metaphysics, as its name suggests, deals with matters that are beyond the realm of experience. It either deals with concepts that lie outside of experience (like God) or it deals with the totality of possible experience (like whether the world has a beginning and an end). Intuition and understanding are of no use here. Metaphysics deals with the faculty of pure reason, and the ideas contained therein.

The distinction between the understanding and reason is crucial. Philosophical error frequently arises from a confusion of one for the other. Any concept that can be applied to experience belongs to the faculty of understanding and has nothing to do with metaphysics. Reason is not directed toward experience, and any attempt to apply the ideas of reason to experience is mistaken.

Reason tries to make experience complete. Reason tries to tie all of experience together and to give it meaning. This drive to metaphysics is not in itself problematic; it becomes wrong-headed only when we apply our pure intuitions or pure concepts of the understanding to the pursuit.

Kant distinguishes three different kinds of "ideas of reason"— psychological ideas, cosmological ideas, and the theological idea—that between them contain all of metaphysics. This summary will deal with psychological ideas, while the summary of sections 50–56 will deal with cosmological and theological ideas.

Psychological ideas try to identify some sort of substance or ultimate subject underlying all the predicates we can apply to a subject. For instance, we can describe a cat as "a thing with claws" or "a thing that purrs" and so on, but what is the "thing" itself? What do we have left over when we peel away all the predicates? Kant suggests that this search is futile: the understanding helps us make sense of experience by applying pure concepts to empirical intuitions, and concepts take the form of predicates. The only knowledge we can have comes in the form of predicates attached to subjects.

A possible candidate for ultimate subject comes in the form of the thinking ego, or soul. When describing internal states ("I think," or "I dream," for example), we refer back to an "I" that is fundamental, indivisible, and unique. However, Kant argues, this "I" is not a thing or a concept that we can have knowledge of in itself. That we are capable of experience at all suggests that we have some sort of consciousness, but we refer to this consciousness (or soul) without having any substantial knowledge of it.

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