It may be useful to have the following excerpts from my work, as far as I had taken it two years ago, titled "Dewey and Peikoff on Kant's Responsibility." I would expect that some of this supports Peikoff's views on Kant in DIM and some of it undermines his picture.
Dewey . . . is not saying that Germany’s marvelous advances in science, industry, and the military were caused by Kant. He means, rather, “that Kant detected and formulated the direction in which the German genius was moving, . . . [and] that his formulation has furnished a banner and a conscious creed which in solid and definite fashion has intensified and deepened the work actually undertaken” (GPP 28-29).
Concerning advances made by German scientists, that last statement by Dewey floats vaguely near the truth in the case of biology to the 1840’s (Lenoir 1982; Richards 2002, 229–61, 309–12, 407–9, 427–30, 488–91; Richards 2008, 31–36, 120–22, 459–60). However, what influence Kant had on German biology in the nineteenth century was mainly on account of his general characterization of organism life and his characterization of biological understanding in Critique of Judgment. His characterization of teleological judgments as strictly regulative (virtually hypothetical and immutably so), a doctrine well suited to his critical and transcendental idealism, was immediately pushed aside by German biologists. They took Kant’s teleological judgment to be essential in their advancing discipline, but they took teleology to be, in one form or another, a governing principle really operating in the physical domain of life, independently of our requirements for scientific explanation.
Advances in German science were not on account of Kant’s particular form of idealism and his situation of science within it. In the case of chemistry and physics, in Germany after Kant, his doctrines sometimes furnished “a banner and a conscious creed,” but it is not the case that they “intensified and deepened the work actually undertaken.”
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Dewey is on the right track in stressing the repercussions of the obsessive a priori in Kant’s philosophy. I should say, however, that it is not plausible that the rational organization of concepts for research by the great German physicists in the nineteenth and early twentieth centuries was in accord with the a priori gloss Kant attempted in his pretentious rewrite of Newtonian mechanics in Metaphysical Foundations of Natural Science (1786). Helmholtz, Mach, Einstein, and other luminaries of physical science had their particular stands on Kant. But Kant’s claim of the a priori nature of such things as the principle of inertia was silliness to them as it is to us. In his elegant text on mechanics, Hertz (1899) gave steady supplication to the Kantian a priori character of the kinematical prerequisites of dynamics, but he really had reverted to the pre-Kantian notion of the a priori. Although Hertz took what we would now call Galilean kinematics to be insusceptible to empirical test for correctness, he maintained that its value or worth hangs on whether the dynamics in which it is used coincides with experience. There is no such vulnerability to experience for Kant’s a priori principles of dynamics. In stark contrast to Kant, Hertz regarded the principle of inertia, his Fundamental Law of dynamics, to be “inferred from experience” (1899, 144). . . .
Dewey’s 1915 effort to find philosophical roots of Germany’s disastrous attitudes and behavior in the sharp differences between the critical philosophy of Kant and his own philosophy of pragmatic experimentalism (or empiricism) goes overboard. Dewey thinks dangerous “the mental habitudes generated by attachment to a priori categories” (GPP 40), dangerous that “no moral, social, or political question is adequately discussed in Germany until the matter in hand has been properly deduced from an exhaustive determination of its fundamental Begriff [concept] or Wesen [essence]. Or if the material is too obviously empirical to allow of such deduction, it must at least be placed under its appropriate rational form” (GPP 41–42). It is plain that “the whole modern liberal social and political movement has allied itself with philosophical empiricism” (GPP 44). Think of Locke or Mill. “A hierarchically ordered and sub-ordered State will feel an affinity for a philosophy of fixed categories, while a flexible democratic society will, in its crude empiricism, exhibit loose ends” (GPP 44).
It might be argued—and this may be an underground assumption of Dewey’s—that Kant saved rationalism and idealism in Germany, by his original and influential reform of them together with his mighty counter to empiricism and materialism. It is plausible that without Kant’s critical and transcendental idealism, the idealisms of Fichte, Schelling, and Hegel would not have come about. Nevertheless, I expect these three men would have made philosophies, even had Kant never existed. Without Kant, these three might well have made new ways in philosophy, and it is reasonable to suppose they would have sprung their creations not from the empirical tradition in Germany (Sassen 2000; Kuehn 1987), but from the Leibnizian-Wolffian tradition. Then too, it is plausible that these three would have had the relations they did have to the Romantic movement in Germany. Goethe and Schiller were happening, Kant or no. Goethe was a realist, not an idealist. He put spirit into nature and that nature into man. Goethe’s only reliance on Kant was in biology, and like the other German biologists, unlike Kant, he took teleology to be in nature. The German enlightenment, the fideist reactions to it, and the German romantic movements in the arts and philosophy, would have happened without Kant.
It is true that Kant’s a priori elements yield a grand organized structure of the phenomenal world. But it is not a resulting organization any more grand and unified than was taught by Kant’s contemporaries such as the Leibnizians Moses Mendelssohn and Johann Eberhard or than would be taught by a Thomist philosopher anywhere in the world. . . .
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More important than systematic character in philosophy, whether a priori or entirely grounded by experience, would be Kant’s refashioning of the concept of the a priori. William Tait (1992) has argued persuasively: for Leibniz and Plato the a priori discipline of geometry had been the study of a type of structure whose truths hold independently of whether that structure is exemplified in the physical world; for Kant a truth of geometry is a priori true of the physical world. This facilitates casting the fundamental organization of the phenomenal world as issuing from something deep inside mind.
Fichte, Schelling, and Hegel inherit and transform the Kantian a priori power of structuring the world, for use in their various editions of idealism, their various versions of Innerlichkeit. Dewey claims that “the Germans, more readily than other peoples, can withdraw themselves from the exigencies and contingencies of life into a region of Innerlichkeit [inwardness, or subjectivity; profoundness]” (GPP 46). We shall evaluate this claim in the sequel.
Greatest within any possible importance of Kant’s a priori for the historical nightmare of the Great War is the role of the a priori in Kant’s renovation and central placement of duty in his ethics. That ethical realm, in Kant’s picture, is the realm of inner rational being. It is shielded from fundamental bases in the phenomenal world, outer and inner, by Kant having rendered the basic form of the phenomenal world fixed a priori by the sentient subject and by having free will completely compatible with the phenomenal world even while accepting that no free will is possible in the phenomenal world alone. The fact of free will is a testimony of the existence of the noumenal world, a touch of it within the rational self.