  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.