This paper considers a principal concept of metaphysics – the category of substance – as it figures in Kant’s critical program of establishing metaphysics as a science. Like Leibniz, Kant identifies metaphysical concepts through logical reflection on the form of cognitive activity. He thus begins with general logic’s account of categorical judgment as an act of subordinating predicate to subject. This categorical form is then considered in transcendental logic with reference to the possibility of its real use. Transcendental reflection reveals that the categorical form, in its potential for such use, constitutes the category of substance and accident, representing a first real subject and a determination of its existence. But to qualify for objective, scientific employment, metaphysics’ concepts must admit of real definitions, which show their objects to be possible, and such possibility, pace Leibniz, can be established only in relation to possible experience. Thus, relying on his doctrine of the schematism, Kant shows the category to figure constitutively in experience, as the ground of the first law of nature, that in all change substance persists.