In this chapter, Annelie Grosse elaborates on the relationship between experience and rational knowledge in eighteenth-century psychology, a discipline that emerged in the wake of Wolff’s empirical psychology. After a brief discussion of Wolff’s conception of the interplay between a priori and a posteriori sources of knowledge in his German Metaphysics, Grosse examines how Jean Henri Samuel Formey, the permanent secretary of the Berlin Academy and francophone popularizer of Wolff’s writings, appropriated the latter’s empirical psychology. In his Essay on Dreams (1746) Formey explained the phenomenon of dreaming by elaborating a complex concept of experience that drew on the methodological principles of Wolffian empirical psychology but, unlike Wolff’s metaphysical approach, did not rely on everyday experiences alone but also on physiological descriptions of the nervous system. As such, Grosse argues, Formey not only undercut the strict separation between metaphysics and natural philosophy characteristic of Wolff’s philosophical system but also blurred the divide between Wolffian rationalism and Newtonian empiricism in mid-eighteenth-century Germany.