During the development of psychiatric nosology during the nineteenth century, faculty psychological systems, often proposed by philosophers, played a substantial role in the development of psychiatric diagnostic systems. The most important of those faculties proposed were cognition and emotion. Because patients often displayed dysfunction in multiple faculties, alienists had to understand how these faculties interrelated. To address this question, nineteenth-century psychiatric diagnosticians postulated causal relationships based on folk criteria of understandability (although longitudinal observations were also utilized). This history puts efforts to apply hard reductionist models to the resulting psychiatric diagnostic categories in a conundrum. The naïve inductivist view of the origins of our psychiatric diagnostic categories – that they arose from raw observations by great skilled clinicians of the past – is untenable. Rather, the road from clinical observations to diagnostic categories passed through both theories of mental functioning (aka faculties) and hypotheses of mental causation between different disordered faculties.