Perhaps it would not be an exaggeration to say that there is no system of thought more intimately bound up with one fundamental principle than is the system of Hegel. Even a cursory reading of his works is sufficient to convince one that the doctrine of the Notion, whatever it may be, is basic to the system; and a more detailed study only forces the conviction home. In the Phenomenology, in the Encyclopaedia, in the History of Philosophy, in the Philosophy of Religion, everywhere it is this doctrine of the Notion upon which emphasis is laid. Indeed, if one were to say that the entire system is just the explication of this doctrine, its elaboration by definition and application, one would be well within the bounds of justification. A correct interpretation of the system, consequently, depends upon a thorough comprehension of the doctrine of the Notion; if this doctrine is neglected, the system must remain a sealed book. The aim of the present monograph is to set forth this doctrine of the Notion, to emphasize its importance for a theory of knowledge, and, in the light of it, to give some insight into Hegel's conception of ultimate reality.