 While the soul, as a simple substance, is understood to be a part of the world, and so is implicated in the treatment of cosmology, this does not exhaust what can be known of it, a fact that leads Wolff to treat it as a separate topic of metaphysics. Indeed, Wolff’s psychology constitutes one of his signal and most historically influential innovations. Most generally, insofar as Wolff seeks to offer a scientific account of the human soul specifically, and indeed with a focus on its cognitive and conative functions, his psychology represents a significant, and distinctly modern, departure from both the treatment of the soul in the context of a generic science of living things, still prevalent among Aristotelian natural philosophers in seventeenth century Germany, and from the metaphysical treatment of the soul in the context of a pneumatology, or doctrine of finite and infinite spirit (Stiening 2003, Vidal 2011). More narrowly, Wolff’s principal, and best-known innovation in psychology consists in his clear separation between two distinct investigations of the soul, the first based on the observation of one’s own mind, identified as empirical psychology, and the second which seeks to use reasoning to uncover truths about the soul that are not readily disclosed by experience, identified as rational psychology.