A product partly of Naturphilosophie and partly of the eighteenth-century tradition, a physiological psychology of the unconscious rose to prominence between 1820 and 1850. Its central idea was that there is an unconscious mind that is constituted by our physiology. Consciousness is only the tip of the psychological iceberg, as it were, whose submerged bulk, supporting and shaping the smaller conscious part, is an assemblage of drives and needs created by the physiological processes of the body. This model of the physiological unconscious is a continuation of the tradition that we have been tracing: the theory of dark ideas, the plastic imagination, the vitalistic physiology of Herder and Goethe, the Idealist and Romantic distinction between conscious and unconscious realms, the physiology of the Naturphilosophen. The relation of the unconscious to rationality is, as one might expect from its ancestry, ambivalent. The unconscious is rationality's enemy: it undermines reason's attempt to govern according to ideas. At the same time, the unconscious is a product of physiology and is therefore natural and intelligible. It exemplifies L. L. Whyte's claim that ‘the discovery of the unconscious is the recognition of a Goethean order, as much as of a Freudian disorder, in the depths of the mind’. The focus of the present chapter is on three (in other respects quite different) exponents of this form of psychology: Arthur Schopenhauer, Carl Gustav Carus, and Georg Büchner. They represent variations on the same theme.