“We think a triangle as object in so far as we are conscious of the combination of three straight lines according to a rule by which such an intuition can at all times be generated. This unity of rule determines the whole manifold and limits it to conditions which make the unity of apperception possible; and the concept of this unity [of rule] is the representation of the object.... All knowledge demands a concept, ... and a concept is always, as regards its form, something general, something that serves as a rule. Thus the concept of body serves as a rule to our knowledge of outer appearances, in accordance with the unity of the manifold which is thought through it.... The concept of body necessitates ... the representation of extension, and therewith of impenetrability, shape, etc.”[775] Such is the manner in which Kant accounts for our concept of the transcendental object. It consists of two main elements: first, the notion of an unknown _x_, to which representations may be referred; and secondly, the consciousness of this _x_ as exercising compulsion upon the order of our thinking. The former notion is framed on the pattern of the transcendental subject; it is conceived as another but unknown subject. The consciousness of it as a source of external necessity is mediated by the empirical concepts which transcendental apperception also makes possible. And from this explanation of the _origin_ of the concept of the transcendental object Kant derives the proof of its _validity_.[776] It is indispensable for the realisation by the unitary self of a unitary consciousness. “This relation [of representations to an object] is nothing else than the necessary unity of consciousness, and therefore also of the synthesis of the manifold, by a common (_gemeinschaftlich_) functioning of the mind, which unites it in one representation.”[777] Through instruments empirical in origin, and subjectively necessary, the notion of an objective necessity is rendered possible to the mind.