Kant’s diagnosis regarding the difficulties of metaphysics in proceeding along the secure path of science is well-known. While Kant’s original criterion for determining the scientific nature of knowledge is the synthetic a priori judgment, there is another important criterion he shares with his predecessors such as Wolff and Meier: it concerns the systematic character of knowledge. The Lectures on metaphysics reveal that the discussion surrounding the systematicity of knowledge leads Kant to the discovery of transcendental illusion: a type of error that is rooted in the very nature of reason and thus takes on the curious form of an a priori error. In this context, Kant’s demand for a clear delineation between the different sciences is also very relevant, as it leads him to identify the specificity of metaphysics as a form of a priori knowledge.