  The second chapter of the Metaphysical Foundations, the Dynamics, considers how it is possible to experience matter as filling a determinate region in space. Propositions 1–4 are devoted to exhibiting the nature and necessity of repulsive forces. In Proposition 1 Kant argues that repulsive force is required for matter to fill space, since solidity, understood by “Lambert and others” as the property matter would have by means of “its mere existence” (4:497), cannot truly explain how one matter resists another matter's attempt to penetrate it. Kant then specifies several central features of repulsive forces in Propositions 2 and 3. Repulsive forces admit of degrees to infinity, since one must always be able to think of a slightly greater or lesser force, and although matter can be compressed to infinity, it can never be penetrated, since that would require an infinite compressing force, which is impossible.