In Kant's Metaphysical Foundations of Natural Science are found a dynamist reduction of matter and an account of the communication of motion by impact. One would expect to find an analysis of the causal mechanism involved in the communication of motion between bodies given in terms of the fundamental dynamical nature of bodies. However, Kant's analysis, as given in the discussion of his third law of mechanics (an action-reaction law) is purely kinematical, invoking no causal mechanisms at all, let alone (...) dynamist mechanisms, in the explanation. It is argued that the reason for the non-incorporation of Kant's theory of matter in the account of impact is his overriding desire to provide a mathematical framework to explain the communication of motion, a provision which Kant felt to be impossible in the context of a metaphysical dynamism. (shrink)