Certain modes of knowledge leave the field of all possible experience and have the appearance of extending the scope of our judgments beyond the limits of experience. The unavoidable problems of pure reason are God, freedom and immortality. Mathematics is an example of how far, independently of experience, we can progress in a priori knowledge. Those in which the connection is thought without identity. Judgments of experience are all synthetic.
All mathematical judgments are synthetic.
They are a priori because they are necessary. Metaphysics ought to contain a priori synthetic knowledge. There is a long section on Hume. The subject-matter is the understanding which passes judgment upon the nature of things, and this understanding again only in respect of its a priori knowledge. Transcendental Philosophy is only the idea of a science.