Kant closely associates his transcendental principles of the understanding with certain specific principles of Newtonian physics ¡ª which latter he calls metaphysical principles of pure natural science. Prominent among these are (i) the principle of the conservation of mass or quantity of matter ("in all changes of the material world the quantity of matter remains unchanged" [B 17; compare 4,541]); (ii) the law of inertia ("every change of matter has an external cause" [4,543; compare 5,181]); and (iii) the principle of the equality of action and reaction ("in all communication of motion action and reaction must always be equal" [B17; compare 4,544]).¹ In the Metaphysical Foundations of Natural Science Kant develops a detailed presentation of what he calls pure natural science closely following the table of categories (and therefore transcendental principles) articulated in the first Critique (compare B109-110, including the footnote thereto). In particular, in the third chapter or Mechanics of the Metaphysical Foundations Kant presents the above three laws of Newtonian physics as instances or more specific realizations of the transcendental principles of substantiality, causality, and community: each results from starting with the corresponding transcendental principle from the first Critique and substituting in, as it were, the empirical concept of matter or body articulated in the Metaphysical Foundations.