Just as appearances in the external world suggest to us that there are things in themselves, so inner sensations suggest to us that we have some sort of soul or ego. But, just like things in themselves, we can know nothing about this soul; we can know only about the appearances that manifest themselves. This conclusion suggests that Descartes is wrong in thinking we can know about the mind better than we can know about external bodies.
What we can say about the soul we can say only in reference to our own experience. Thus, we can't know whether or not the soul is immortal, since that is to ask questions about the soul outside the realm of experience.
In the Prolegomena, Kant divides mental activity into three major faculties. First, there is the faculty of sensibility that uses the pure intuitions of space and time to form our sensations into empirical intuitions. This faculty helps us organize and make sense of what we see, hear, smell, touch, and taste. We can also use the pure intuitions of space and time to reason mathematically.
Second, there is the faculty of understanding that uses pure concepts to form our empirical intuitions into appearances. This faculty allows to make sense of what we see, hear, smell, touch, and taste as according itself with regular, universal laws of nature, and so helps us make general inferences and conclusions. That, effectively, is the business of natural science
Third, Kant introduces us to the faculty of reason in this part of the text. While the faculties of sensibility and understanding help us make sense of experience, reason helps us make sense of purely mental concepts. It does this by means of ideas, which try to fill out and give completeness to the concepts we apply in experience. For instance, psychological ideas take the concept of substance and try to flesh out what it is we mean when we talk about a "thing." What substance underlies a thing and makes it what it is? We will see similarly that the cosmological ideas try to fill out our concept of cause, trying to identify connections that go beyond those we encounter in experience, and that the theological idea tries to fill out our concept of community, trying to identify what unifies everything that exists.
Our faculty of sensibility gives us math, our faculty of understanding gives us science, and our faculty of reason gives us metaphysics. The important conclusion to draw from this discussion of faculties is that metaphysics is the product of pure reason and deals only with ideas in our head; in other words, metaphysics cannot tell us about how things are in themselves. Metaphysics as Kant conceives it is more a matter of untying mental knots than determining whether or not God exists.