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"; 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.