To the argument supporting this last conclusion Kant does not give the attention which its importance would seem to deserve. He points out that as the given is an unconnected manifold, its unity can be obtained only by synthesis, and that such synthesis must conform to the conditions prescribed by the unity of apperception. That these conditions coincide with the categories he does not, however, attempt to prove. He apparently believes that this has been already established in the metaphysical deduction.[895] The forms of unity demanded by apperception, he feels justified in assuming, are the categories. They may be regarded as expressing the minimum of unity necessary to the possibility of self-consciousness. If sensations cannot be interpreted as the diverse attributes of unitary substances, if events cannot be viewed as arising out of one another, if the entire world in space cannot be conceived as a system of existences reciprocally interdependent, all unity must vanish from experience, and apperception will be utterly impossible.[896] The successive steps of the total argument of the deduction, as given in the first edition, are therefore as follows: Consciousness of time involves empirical self-consciousness; empirical self-consciousness is conditioned by a transcendental self-consciousness; and such transcendental self-consciousness is itself, in turn, conditioned by consciousness of objects. The argument thus completed becomes the proof of mutual interdependence. Self-consciousness and consciousness of objects, as polar opposites, mutually condition one another. Only through consciousness of both simultaneously can consciousness of either be attained. Only in and through reference to an object can an idea be related to a self, and so be accompanied by that self-consciousness which conditions recognition, and through recognition all the varying forms in which our consciousness can occur. From the point of view, however, of a Critical enquiry apperception is the more important of the two forms of consciousness. For though each is the _causa existendi_ of the other, self-consciousness has the unique distinction of being the _causa cognoscendi_ of the objective and _a priori_ validity of the forms of understanding. “The synthetic proposition, that all the variety of empirical consciousness must be combined in a single self-consciousness, is the absolutely first and synthetic principle of our thought in general.”[897]