  In Proposition 2, Kant states his First Law of Mechanics: the total quantity of matter remains the same throughout all changes in matter (4:541). His proof seems to rely (i) on the principle of the First Analogy of Experience that no substance arises or perishes throughout any change in nature and (ii) on the identification of what in matter must be substantial. On this latter point, Kant quickly assumes that the ultimate subject of all accidents inhering in matter must be the movable in space, and that its quantity is the aggregate of the movable in space. In his remark to this proposition, Kant explicitly notes that there is a crucial difference between spatial and non-spatial substances, since the latter, unlike the former, could gradually fade away by degrees. (Kant cites the possibility of consciousness as a concrete example.) Kant uses this difference to argue that since the quantity of matter consists in a plurality of real things external to each other that cannot fade away (as consciousness might), the only way to decrease its quantity is by division.