For Kant, causality is a pure concept of the understanding (under "relation"); it is known a priori and, along with pure forms of intuition (space and time) and other pure concepts of the understanding, makes up for Kant's definition of human experience.

Look into Foucault's reasoning as to why Kant is the beginning of modern philosophy. Yes, Descartes does kick if off with "I think, therefore I am" but Kant develops in his Anthropology and Groundwork the basis of a metaphysical system that is relevant to this day. To my understanding, Foucault attributes this honor to Kant for his writing on how humans understand concepts as distinct from objects, allowing for an abstraction and synthesis beyond the sensible experience of the world.