Abstract: In the Critique of Pure Reason , Kant criticizes Baumgarten’s definition of metaphysics as the science of the first principles of human knowledge ( Metaphysica § 1). He maintains that this definition doesn’t consider the distinction between sensible and pure knowledge. Instead of this definition, Kant provides a new one in the first Critique : metaphysics is the system of pure reason (KrV, A 841/B 869). This definition is first anticipated eleven years before in the Inaugural Dissertation , where metaphysics is defined as pure philosophy (MSI, AA 02: 411). Kant presents the Dissertation as a proof of the propaedeutic that must precede metaphysics, and so anticipates the idea that will be fully developed in the Critique of Pure Reason . In this paper, I will argue that the new definition of metaphysics in the Inaugural Dissertation must be seen as the text’s very important result, establishing the Dissertation ’s status as a propaedeutic to metaphysics and as a specific step in the development of the Critique of Pure Reason .