In the Critique of Pure Reason, Kant repeatedly characterized the thing in itself (Ding an sich or noumenon) in such terms as "the non-sensible cause" of representations or as "the purely intelligible cause" of appear­ances (A 494 = B 522). Again and again he employs the language of causal efficacy with regard to things in themselves. Thus he speaks of "the representations through which they [things in themselves] affect us' (A 190 = B 235) and elsewhere says that things in themselves are in principle unknowable: "they can never be known by us except as they affect us' (Foundations of the Metaphysic of Morals, Ak. 452) because the thing itself is a "transcendental object, which is the cause of ap­pearance and therefore not itself appearance" (A 288 = B 344). The thing in itself is described as "the true correlate of sensibility which is not known, and cannot be known" through its representations (A 30 =B 45).