There must be some third thing, which is homogeneous on the one hand with the category, and on the other hand with the appearance... the schema of the concepts of understanding... This schematism of our understanding, in its application to appearances and their mere form, is an art concealed in the depths of the human soul, whose real modes of activity nature is hardly likely ever to allow us to discover, and to have open to our gaze
Kant, Critique of Pure Reason B181













