In Kant's view, a priori intuitions and concepts provide some a priori knowledge, which also provides the framework for a posteriori knowledge. Kant also believed that causality is a conceptual organizing principle imposed upon nature, albeit nature understood as the sum of appearances that can be synthesized according to a priori concepts.
In other words, space and time are a form of perceiving and causality is a form of knowing. Both space and time and conceptual principles and processes pre-structure experience.
Things as they are "in themselves" — the thing in itself or das Ding an sich — are unknowable. For something to become an object of knowledge, it must be experienced, and experience is structured by the mind—both space and time being the forms of intuition, "Anschauung" in German, (for Kant, intuition is the process of sensing or the act of having a sensation) or perception, and the unifying, structuring activity of concepts. These aspects of mind turn things-in-themselves into the world of experience. There is never passive observation or knowledge.
According to Kant, the transcendental ego — the "Transcendental Unity of Apperception" — is similarly unknowable. Kant contrasts the transcendental ego to the empirical ego, the active individual self subject to immediate introspection. One is aware that there is an "I," a subject or self that accompanies one's experience and consciousness. Since one experiences it as it manifests itself in time, which Kant proposes is a subjective form of perception, one can know it only indirectly: as object, rather than subject. It is the empirical ego that distinguishes one person from another providing each with a definite character.