In the last stage of his last attempt at philosophical work, the “First Fascicle” of the Opus postumum, Kant was apparently trying to unify his theoretical and practical philosophy into a single system of the ideas of nature and freedom. In this work, Kant seems to have wanted to show that the constitution of nature through our forms of intuition and understanding must be compatible with the content of the moral law and our capacity to act in accordance with it, as represented by our idea of God as supreme lawgiver, because both the concept of nature and the idea of God have their common ground in human thought itself. One of the many drafts of a title page that Kant wrote for this never-completed work suggests his intent:

Some commentators have interpreted texts like this to mean that in his final years Kant undertook a radical revision of his previous critical philosophy.