therefore presuppose matter, and matter presupposes space. Strictly speaking, co-existence is a negative determination, which merely signifies that two things or states are not different in respect of time; thus their difference is to be sought for elsewhere. But in any case, our knowledge of the permanence of substance, i.e., of matter, must be based upon insight a priori; for it is raised above all doubt, and therefore cannot be drawn from experience. I deduce it from the fact that the principle of all becoming and passing away, the law of causality, of which we are conscious a priori, is essentially concerned only with the changes, i.e., the successive states of matter, is thus limited to the form, and leaves the matter untouched, which therefore exists in our consciousness as the foundation of all things, which is not subject to becoming or passing away, which has therefore always been and will always continue to be. A deeper proof of the permanence of substance, drawn from the analysis of our perception of the empirical world in general, is to be found in the first book of this 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