work, § 4, where it is shown that the nature of matter consists in the absolute _union of space and time_, a union which is only possible by means of the idea of causality, consequently only for the understanding, which is nothing but the subjective correlative of causality. Hence, also, matter is never known otherwise than as producing effects, _i.e._, as through and through causality; to be and to act are with it one, which is indeed signified by the word _actuality_. Intimate union of space and time—causality, matter, actuality—are thus one, and the subjective correlative of this one is the understanding. Matter must bear in itself the conflicting properties of both factors from which it proceeds, and it is the idea of causality which abolishes what is contradictory in both, and makes their co-existence conceivable by the understanding, through which and for which alone matter is, and whose whole faculty consists in the knowledge of cause and effect. Thus for the understanding there is united in matter the inconstant flux of time, appearing as change of the accidents, with the rigid immobility of space, which exhibits itself as the permanence of substance. For if the substance passed away like the accidents, the phenomenon would be torn away from space altogether, and would only belong to time; the world of experience would be destroyed by the abolition of matter, annihilation. Thus from the share which space has in matter, _i.e._, in all phenomena of the actual—in that it is the opposite and counterpart of time, and therefore in itself and apart from the union with the latter knows absolutely no change—the principle of the permanence of substance, which recognises everything as _a priori_ certain, had to be deduced and explained; but not from mere time, to which for this purpose and quite erroneously Kant has attributed _permanence_. In the essay on the principle of sufficient reason, § 23, I have fully explained the incorrectness of the following proof of the _a priori_ nature and of the necessity of the law of causality from the mere succession of events in time; I must, therefore, content myself here by referring to that passage.(6) This is precisely the case with the proof of reciprocity also, the concept of which I was obliged to explain above as invalid. What is necessary has also been said of modality, the working out of the principles of which now follows.