ving force, that is, how one matter communicates its motion to another by means of its moving force. Kant begins, in Proposition 1, by clarifying how the quantity of matter is to be estimated before stating, in Propositions 2–4, three Laws of Mechanics. After first defining the quantity of matter and the quantity of motion (or, in contemporary terms, impulse, i = mv ), Kant asserts that the quantity of matter, in comparison with every other matter, can be estimated only by the quantity of motion at a given speed (4:537). Kant’s proof proceeds by way of elimination. The quantity of matter, which is the aggregate of the movable in a determinate space, cannot be estimated by counting the number of parts it has, since, as was established in the Dynamics, every matter is infinitely divisible. Nor can one estimate the quantity of matter merely by considering its volume, since different matters can have different specific densities. As a result, the only universally applicable way of estimating the quantity of matter is to hold the velocity of matter constant. 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 consi