Kant's rejection of the rational psychologists' conclusion that the soul is a substance is well known. Less well known is that Kant himself repeatedly asserts that the soul is a substance across his recorded thought, even throughout the 1780s and 1790s. This presentation considers the origins of Kant's First Paralogism in his pre-Critique recorded thought and its development in the Critique and later sources. I argue that Kant's transcendental researches lead him to draw a distinction between phenomena and noumena but also, in turn, between a determinate concept of phenomenal substance, which implies permanence, and an indeterminate yet ontologically significant concept of noumenal substance, which does not imply permanence. For lack of these transcendental researches, which would come to form the basis for Kant's First Analogy and First Paralogism, the rationalists conflate these two concepts of substance and thus mistakenly ascribe permanence to the noumenally substantial soul where Kant does not.