In the seventh paragraph of the Preface to MAN, Kant contends that a discipline contains only as much “proper science” as mathematics is applicable within it (4:470). In the eighth and ninth paragraphs, he uses the examples of psychology and chemistry to illustrate how “improper” sciences can fail to meet this condition.1 The tenth through twelfth paragraphs aim to show that metaphysics is necessary for “proper” natural science (as Kant first asserted in the sixth paragraph), by arguing that metaphysical principles are needed specifically for the application of mathematics. This discussion spans the middle third of the (eighteen-paragraph-long) Preface, and its length and centrality indicate that securing the applicability of mathematics is crucial for Kant’s foundational project.