co-existence of many things constitutes, in fact, the essence of reality, for through it permanence first becomes possible; for permanence is only knowable in the change of something which is present along with what is permanent, while on the other hand it is only because something permanent is present along with what changes, that the latter gains the special character of change, _i.e._, the mutation of quality and form in the permanence of substance, that is to say, in matter.(7) If the world were in space alone, it would be rigid and immovable, without succession, without change, without action; but we know that with action, the idea of matter first appears. Again, if the world were in time alone, all would be fleeting, without persistence, without contiguity, hence without co-existence, and consequently without permanence; so that in this case also there would be no matter. Only through the union of space and time do we reach matter, and matter is the possibility of co-existence, and, through that, of permanence; through permanence again matter is the possibility of the persistence of substance in the change of its states.(8) As matter consists in the union of space and time, it bears throughout the stamp of both. It manifests its origin in space, partly through the form which is inseparable from it, but especially through its persistence (substance), the _a priori_ certainty of which is therefore wholly deducible from that of space(9) (for variation belongs to time alone, but in it alone and for itself nothing is persistent). Matter shows that it springs from time by quality (accidents), without which it never exists, and which is plainly always causality, action upon other matter, and therefore change (a time concept). The law of this action, however, always depends upon space and time together, and only thus obtains meaning. The regulative function of causality is confined entirely to the determination of what must occupy _this time and this space_. The fact that we know _a priori_ the unalterable characteristics of matter, depends upon this derivation of its essential nature from the forms of our knowledge of which we are conscious _a priori_. These unalterable characteristics are space-occupation, _i.e._, impenetrability, _i.e._, causal action, consequently, extension, infinite divisibility, persistence, _i.e._, indestructibility, and lastly mobility: weight, on the other hand, notwithstanding its universality, must be attributed to _a posteriori_ knowledge, although Kant, in his “Metaphysical Introduction to Natural Philosophy,” p. 71 (p. 372 of Rosenkranz’s edition), treats it as knowable _a priori_.