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  • Print publication year: 2004
  • Online publication date: November 2009

6 - Kant's metaphysical proof of the Law of Inertia



One might suggest that Kant's attempt to provide metaphysical foundations for Newtonian mechanics survives, despite problems identified in chapter 5, because his proofs of the laws of motion are independent of his metaphysical theories of matter. While Kant's proofs are to a certain extent independent of his theory of matter, close examination shows that his proof of the law of inertia is fallacious. Moreover, this fallacy extends and reinforces the problems with Kant's dynamic theory of matter. Indeed, this further fallacy shows that Kant's transcendental and metaphysical efforts to justify even commonsense causal judgments about physical events fail. This implies that causal judgments cannot be justified, legitimated, or shown a priori to be “objectively valid” by Kant's transcendental idealism. This Critical failure strongly supports and reinforces the plausibility of philosophical realism. In chapter 7 I argue that Kant's transcendental method and proof strategy provide richer resources for justifying realism and causal judgments than Kant recognized, resources that can dispense with transcendental idealism.


According to Kant's Foundations, a science, properly speaking, must be organized on rational principles (M 467–8). Any proper science has a pure rational a priori part, its metaphysical foundation (M 469–70; above, §§43, 48). After writing the Foundations (1786), Kant claims, in the second edition preface to the first Critique (1787), that his transcendental idealist account of time explains the possibility of the synthetic a priori cognitions involved in the universal theory of motion (B49, 3:59.14–16).

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