  In the Preface to the Metaphysical Foundations Kant claims that chemistry, at least as he understood it in 1786, was not science “proper”, but such a claim leaves open the possibility that chemistry could be fully scientific in some other sense or that, with time, it could develop into science proper. Up through 1787, Kant accepted the fundamental tenets of Stahl's chemical theory, according to which water and air are fundamental elements that function as vehicles for change in both inflammable and “earthy” substances, and he commented on particular issues in chemistry in his various physics lectures. As a result, it is clear that Kant considers chemistry to be a science in some sense even during the Critical period. However, beginning in the mid-1780s (and extending through the mid to late 1790s), Kant becomes aware of significant new developments in chemistry (as evidenced in the Danziger Physik and documented in Lichtenberg's revised, third edition of Erxleben's Anfangsgründe der Naturlehre from which Kant lectured). In particular, Kant comes to reject Stahl's theory, favoring Lavoisier's anti-phlogistic account of combustion and calcination, which relied on his doctrines of latent heat and the caloric theory of the states of aggregation. While Kant never explicitly claims that chemistry, so understood, can be considered science proper, Kant's interest in these issues in the Opus postumum suggests that he was optimistic about providing the kind of foundation that would be required for it to attain this status.