Kant's Transcendental Idealism
Immanuel Kant (1724–1804) famously wrote that his "transcendental idealism" arose in response to the radical skepticism of David Hume (1711–1776; see Prolegomena, p. 260). Unlike Berkeley, Hume doubted not only the independent existence of material objects but even the objective validity of concepts that still remained central to Berkeley's immaterialist system, such as the concepts of causality and God. Kant recognized that these and other metaphysical concepts could be neither verified nor falsified by recourse to experience alone; however, Kant did not simply reject metaphysics (as Hume had done) but sought to determine the legitimacy and scope of metaphysics by asking the prior question of what reason might justifiably claim to know a priori (that is, independent of all experience).
For Kant, the question of the legitimacy and scope of metaphysics is intimately linked to the question of the possibility of "synthetic apriori judgments." As synthetic, such judgments extend our knowledge beyond our mere concepts of things, and as a priori, they have necessary and universal validity. Prior to Kant, the empiricists had argued that all synthetic judgments must be a posteriori (that is, based on experience), while Leibniz and Leibnizians had argued that even seemingly synthetic judgments are not really synthetic, because all the predicates belonging to any particular thing can in principle be discovered through an analysis of the mere concept of the thing. The Leibnizian option was unacceptable to Kant, because it entailed that human sensibility is not essentially different from (but is simply a confused form of) human understanding, and thus that human knowing is different in degree, but not in kind, from divine knowing. The empiricist option was unacceptable because, for Kant, judgments based on experience (a posteriori judgments) could never yield knowledge about what is necessarily and universally the case. Against both sides, Kant argued that synthetic a priori knowledge is possible for us, because we possess a kind of sensibility (or intuition) that is not merely empirical (a posteriori) but a priori. More specifically, we possess the a priori forms of intuition—space and time—where space is the form of all outer sense, and time is the form of all inner sense. For Kant, no object can be given to us (and thus we can have no access to objects beyond our mere concepts), except through the a priori forms of space and time, which are the "subjective conditions" of our own mode of intuiting things. Kant also argued that we possess a priori concepts or "categories" of the understanding which—like the a priori forms of intuition—are not derived from experience but rather which make our experience of objects possible in the first place. Indeed, Kant argues, there would be no such thing as "objects" for us if we did not make judgments applying our own a priori concepts (or categories) to the sensible manifold that is intuited by us through our own a priori forms of space and time. Kant concludes that the "objects" we know through the a priori forms of intuition and categories of the understanding are not "things-in-themselves" (they are not things as they might exist apart from our own a priori conditions of knowing) but only appearances.
Kant's denial that we can have knowledge of "things-in-themselves" is not meant to imply that the empirical objects of ordinary experience (what Kant calls appearances) are "un-real" or merely illusory. For Kant, the objects of ordinary experience are certainly real, for "the real" is simply that which exercises some degree of influence on our sensibility (Critique, A 165; B 208). But while objects of ordinary experience are empirically real, Kant insists that they are "transcendentally ideal" (and not transcendentally real), which is to say that they are not to be identified with anything beyond—or anything that transcends—the bounds of possible experience or the a priori subjective conditions that make such experience possible in the first place. Simultaneously embracing both "transcendental idealism" and "empirical realism," Kant claims to have shown how we are justified in making synthetic a priori knowledge claims and in employing concepts that are neither derived from nor verified through experience. But just as Kant's transcendental idealism entails the distinction between things-in-themselves and appearances, it also entails a distinction between the legitimate and illegitimate employment of pure (a priori) reason. For Kant, we can legitimately pursue a limited "metaphysics of experience," and we can legitimately make synthetic a priori knowledge claims about objects of possible experience. For example, the concept of "causality" (even though "pure" and underived from experience) remains objectively valid when applied to things that can be intuited by us under the a priori conditions of space and time. But Kant also argues that we cannot legitimately pursue metaphysics or make synthetic a priori claims regarding objects that transcend all possible experience. Furthermore, he argues that the attempt to make knowledge claims about the non-sensible objects of traditional metaphysics (for example, God, the soul, and the world as a whole) inevitably leads reason into illusion and self-contradiction. But while we cannot obtain objectively valid theoretical knowledge of such non-sensible objects, our ideas regarding such objects (for example, our idea of God) may continue to play a legitimate role in guiding our search for complete knowledge in our theoretical pursuits and the complete good in our moral pursuits.
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