  In the General Remark to Dynamics Kant addresses two main issues. First, Kant considers how it is that the specific varieties of matter (e.g., water as different from mercury) might be reduced, at least in principle, to the fundamental forces of attraction and repulsion. The second issue concerns the fundamental distinction between the “mathematical-mechanical” and the “metaphysical-dynamical mode of explanation”. The former mode of explanation, which is associated with the postulation of atoms and the void, employs nothing more than the shapes and motions of fundamental particles and empty interstices interspersed among them. It contrasts with the metaphysical-dynamical mode, which employs fundamental moving forces (e.g., attraction and repulsion) in its explanations. Kant grants that the mathematical-mechanical mode has an advantage over the metaphysical-dynamical mode, since its fundamental posits can be represented (indeed, “verified” (4:525)) mathematically, whereas he repeatedly admits that the possibility of fundamental forces can never be comprehended, i.e., their possibility can never be rendered certain.  However, Kant thinks that this advantage is outweighed by two disadvantages. First, by presupposing absolute impenetrability, the mathematical-mechanical mode of explanation accepts an “empty concept” at its foundation. Second, by giving up all forces that would be inherent in matter, such a mode of explanation provides the imagination with more freedom “than is truly consistent with the caution of philosophy” (4:525).