  Whatever its form, the content of the Opus postumum includes reflections on a series of important topics in physics. Three clusters are particularly noteworthy. (1) Kant develops more detailed views about a number of outstanding issues concerning matter theory that he had discussed (often in a tentative way) in the Metaphysical Foundations, such as fluidity, rigidity, cohesion, and the quantity of matter. (2) Kant argues for the existence of an all-encompassing ether. This might seem to be a natural development, since the Metaphysical Foundations was non-committal on the point, but what is surprising is that Kant thinks that the ether can be established a priori (e.g., 21:222), which might seem to conflict with Kant's project in the Critique of Pure Reason (or with his description of his position as “formal idealism,” 4:337). (3) Kant also explores the idea that the subject must posit itself in positing the various forces in matter, a doctrine that has come to be known as the Selbstsetzungslehre, and attempts to incorporate it into his views on how man is situated in between the world of experience and God, whose existence is a central requirement of morality.