Secondly, the real in space—that is, matter—is conditioned. Its internal conditions are its parts, and the parts of parts its remote conditions; so that in this case we find a regressive synthesis, the absolute totality of which is a demand of reason. But this cannot be obtained otherwise than by a complete division of parts, whereby the real in matter becomes either nothing or that which is not matter, that is to say, the simple. Consequently we find here also a series of conditions and a progress to the unconditioned. Thirdly, as regards the categories of a real relation between phenomena, the category of substance and its accidents is not suitable for the formation of a transcendental idea; that is to say, reason has no ground, in regard to it, to proceed regressively with conditions. For accidents (in so far as they inhere in a substance) are co-ordinated with each other, and do not constitute a series. And, in relation to substance, they are not properly subordinated to it, but are the mode of existence of the substance itself. The conception of the substantial might nevertheless seem to be an idea of the transcendental reason. But, as this signifies nothing more than the conception of an object in general, which subsists in so far as we cogitate in it merely a transcendental subject without any predicates; and as the question here is of an unconditioned in the series of phenomena—it is clear that the substantial can form no member thereof. The same holds good of substances in community, which are mere aggregates and do not form a series. For they are not subordinated to each other as conditions of the possibility of each other; which, however, may be affirmed of spaces, the limits of which are never determined in themselves, but always by some other space. It is, therefore, only in the category of causality that we can find a series of causes to a given effect, and in which we ascend from the latter, as the conditioned, to the former as the conditions, and thus answer the question of reason. Fourthly, the conceptions of the possible, the actual, and the necessary do not conduct us to any series—excepting only in so far as the contingent in existence must always be regarded as conditioned, and as indicating, according to a law of the understanding, a condition, under which it is necessary to rise to a higher, till in the totality of the series, reason arrives at unconditioned necessity. There are, accordingly, only four cosmological ideas, corresponding with the four titles of the categories. For we can select only such as necessarily furnish us with a series in the synthesis of the manifold. 1 The absolute Completeness of the COMPOSITION of the given totality of all phenomena. 2 The absolute Completeness of the DIVISION of given totality in a phenomenon.