The author commences by a vindication of the Scottish philosophy, that he may legitimately build upon substance as a reality as well as phenomenon. He then shows that just as the idea of substance presents, a priori, an alternative, viz., those which are wholly fixed, the subject of law imposed from without only, and those which possess a power of changing their mode of being from within themselves; so, in point of fact, common observation has recognised in all ages two orders of substances or beings characterized by this difference, and filling up in the sphere of reality this alternative given by pure thought; the one being the ground of all natural phenomena, and ordinarily designated body; the other the ground of all moral and intellectual phenomena, and designated spirit.