Aristotle is credited with the first full-fledged robust philosophical discussion and presentation of substance. His account of substance presents different notions of substance, which were elaborated on and modified in the medieval and modern periods. Among those that elaborated on the conception of substance in the modern period are Rene Descartes, John Locke, Baruch or Benedict de Spinoza, George Berkeley, Gottfried Leibniz, David Hume, and Immanuel Kant. What is the nature of substance and how is it understood by these philosophers? In this paper I examine the notions of substance in the philosophical systems of Locke, Spinoza, and Kant. I go beyond this comparative and exploratory exercise to show why Kant takes on a more expansive notion of substance. In particular, for Kant the conceptions of substance we find in Locke and Spinoza do not allow the idea of substance to do the work that substance as a pure concept of the understanding should do.