In the early modern period of philosophy, the study of metaphysics was approached with the assumption that reason alone is sufficient for delivering necessary truths about reality. This assumption was later undermined by David Hume who brought the rational justification of any metaphysical claims that extend beyond experience into question. Hume’s philosophy thus posed a serious challenge to the possibility of a priori metaphysics. Immanuel Kant’s transcendental idealism addresses Hume’s challenge by reconceiving metaphysics within the a priori conditions required for experience to exist at all. Transcendental idealism succeeds in this regard, but Kant’s use of the term metaphysics invites interpretation as to whether or not it is used correctly.
In the rationalist tradition that preceded Kant, it was widely accepted that reason alone could reveal necessary universal truths about the structure of reality using intuition and deduction. (Markie and Folescu 2023). In mathematics, it can be immediately intuited that 2 + 2 = 4, and from this intuition, further mathematical truths can be deduced. Since mathematical truths delivered by intuition and deduction are necessarily true, rationalists extended the same reasoning to metaphysical ideas believing that reason can reveal necessary truths about the structure of reality. This belief was dramatically defeated by Hume’s critique of the idea of necessity. Hume accepted that a priori truths such as those in mathematics, geometry and logic are necessarily true as relations of ideas but necessity cannot be established as an objective feature of the world. According to Hume, knowledge of the objective world can only be grounded by sense experience (Morris and Brown 2023). Hume argued that necessity is never directly experienced, and what are often interpreted as necessary connections are nothing more than habitual customs supplied by the imagination. For Hume, the only beliefs that are rationally justified are those that are justified by direct sense experience or a priori relations of ideas. Since traditional rationalist metaphysical concepts cannot be observed in sense experience, Hume asserted that they should be discarded. Kant took Hume’s boundary of restricting metaphysical cognitions to sense experience as an inescapable problem. Transcendental idealism is Kant’s attempt to use pure reason to determine the possibility, conditions, and limits of a priori metaphysics, while maintaining experience as the boundary of what can be known.
Transcendental idealism is a system of a priori metaphysics with a crucial caveat: all possible a priori metaphysical judgments are limited to appearances, and things-in-themselves, or noumena, are unknowable. While remaining true to Humean position that all knowledge is grounded in experience, Kant’s key innovation is to extend a priori knowledge to include the necessary conditions that must be satisfied in order for experience of an object to occur (Rohlf 2024). The fact that we experience objects presupposes structures that sensation alone cannot supply. Any condition that must be presupposed by experience cannot itself be derived from experience. Therefore, the mind must contribute a priori conditions that make such experience possible. While a priori, the knowledge of the mind’s contributions to the experience of an object is not analytic in the manner of Hume’s relation of ideas; rather, it is synthetic, as it reveals new knowledge not contained in experience itself.
Kant’s inquiry into the necessary conditions of experience reveals two forms through which the mind structures experience. By analysing the conditions that bring about the bare possibility of experience, Kant formulates the forms of intuition, and by assessing what must be the case for the experience of objects, he formulates the forms of understanding. The forms of intuition are the most basic conditions under which experiences can be given. Kant argues that no experience would be possible without the mind providing spatiotemporal ordering. Objects could not be represented in such a way to appear outside of us or next to one another unless space is presupposed by such appearances. For Kant, space represents the form of ‘outer intuition’. Conversely, the temporal ordering of experiences such as succession and duration presuppose time as the form of inner intuition. While the forms of intuition are necessary conditions of experience, they are not sufficient for accounting for all that experience encompasses. Specific objects that are given by sensibility require additional faculties in order for a precise and detailed experience of them to be coherent.
The forms of understanding are articulated as twelve a priori categories which include concepts prominent in rationalist metaphysics such as necessity, causation and substance. Kant identifies these categories as necessary conditions for the coherence of objective experience. Unlike typical rationalist thinkers, Kant does not treat the concepts of the categories as metaphysical claims about things in themselves; instead, the categories are concepts that the understanding supplies to experience, which makes a unified objective experience possible. Without the categories, Kant argues that experience would be disconnected and unintelligible. Since the objects that we experience are represented with precise clarity and determinate structural features, and since the orderliness of such representation cannot come from things-in-themselves, Kant concludes that the mind must contribute the structuring principles to objects. In this way, Kant relocates metaphysics from things-in-themselves to the a priori conditions that make objective experience possible.
While Kant uses metaphysical concepts from the rationalist tradition to explain how the mind unifies object experience, it is not clear whether these particular concepts are necessarily imposed by the mind onto experience, or that they have been merely chosen from a convenient library. It is possible that the real ordering processes are much more complicated, and not so easily explained in terms of concepts. It is also possible that they cannot be articulated at all. Regardless, the question of exactly how the mind structures object experience is less important than the simple realisation that, one way or another, the mind does. If the categories are ill-conceived, the conclusion that the mind contributes structure to object experience a priori remains unaffected, but at the expense of a lower-resolution version of transcendental idealism.
The question of whether the a priori conditions of experience amount to metaphysics hinges on what is considered an acceptable definition of metaphysics. Transcendental idealism is undoubtedly an exhaustive inquiry in the a priori conditions of experience. Kant does not introduce anything beyond what is given by experience and arrives at his complete system organically by investigating only what must necessarily be the case for experience to exist at all. While Kant positions these conditions as metaphysics, whether or not this is correct is a matter of semantics. Transcendental idealism could appropriately be called a theory of the a priori conditions under which experience is possible while either denying the possibility of metaphysics entirely, or restricting the term to speculations that go beyond what experience can justify.
Bibliography
Markie, Peter, and Mădălina Folescu. 2023. “Rationalism vs. Empiricism.” The Stanford Encyclopedia of Philosophy, Fall 2023 Edition. Edited by Edward N. Zalta. Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/entries/rationalism-empiricism/
Morris, William Edward, and Charlotte R. Brown. 2023. “David Hume.” The Stanford Encyclopedia of Philosophy, Fall 2023 edition. Edited by Edward N. Zalta. Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/entries/hume/
Rohlf, Matthew. 2024. “Immanuel Kant.” The Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta, Summer 2024 edition. Metaphysics Research Lab, Stanford University. https://plato.stanford.edu/entries/kant/