Kant demonstrates that we can have rational grounds for holding a belief in the existence of supersensible things that we can never know with certainty. I argue that at the basis of this accomplishment lies a hitherto neglected part of Kant's philosophy: the novel approach Kant brings to the understanding of modalities (possibility, actuality, and necessity). This approach consists in defining modalities in terms of theoretical and practical relations between objects and the subject rather than the ways objects are.

Until the mid1760's, Kant's views on modalities do not display a deep breakthrough from the prevalent conception in the "school" metaphysics, but rather a revisionist approach. Not yet opposed to conceiving modalities in ontological terms, Kant rejects only the inclusion of modalities as predicates in the concepts of things. Yet, from the mid1760's onwards, we see in Kant a shift toward an understanding of modalities in more epistemological terms, as pertaining to the relationship between the epistemic subject and objects.

Kant's early revisionist critique develops into a revolutionary theory of modality in the Critique of Pure Reason. This theory introduces a subjectivist perspective according to which to ascribe a modal status to an object is to posit it in relation to the cognitive faculty of the subject, according to the conditions of experience. This new perspective rules out the possibility of any theoretical proof of the existence of supersensible objects such as God, freedom and immortality, thus undermining the cardinal claims of traditional metaphysics.

Interestingly enough, this conception of modality enables Kant to reestablish these claims in a different domain. In the Critique of Practical Reason , Kant introduces a notion of "practical cognition," which relies on the ascription of modal categories to objects in accordance with their necessary connections to the moral law. It is in virtue of this use of modality that freedom, God and immortality are postulated to be actual, but only as articles of belief rather than knowledge. Therefore, Kant's treatment of modality plays an indispensable role both in his theoretical rehabilitation of traditional metaphysics and in his construction of a new metaphysics upon practical premises.