Kant’s definition of substance is the focus of this philosophical study. The analysis shows that an adequate understanding of the term and the“critical” metaphysics of the substance which result from it must take into account the philosophical controversy over the simple substance in the first half of the 18th century dominated by Gottfried Wilhelm Leibniz and Christian Wolff. It is only against this background that the complexity and inconsistence of the“critical” concept of substance becomes fully apparent.