In Observations on Man, His Frame, His Duties, and His Expectations David Hartley (1749/1971) presented a systematic, comprehensive, complex, and medically informed psychological treatise, drawn from Newtonian mechanics. Evidently motivated by religious beliefs about the perilous state of humankind, he speculated that human nature’s physical foundation in vibrations and association ranged from sensory processes and simple ideas to sympathy (i.e., benevolent social relations, leading to perspective-taking), theopathy (i.e., loving union with God), and moral sensibility (i.e., reliance on moral principles to guide conduct). However, typical accounts of scientific psychology’s roots in Enlightenment thought have neglected the complex psychological processes and developmental, interpersonal, societal, religious, and moral aspects of Hartley’s system. For him, manifestations of sympathy, theopathy, and moral sensibility are central to human experience, whereas self-fulfillment results from the developmental transit of self-interest to moral sensibility. Thus, after describing the multiple facets of association in sensation, ideas, action, language, and memory, I show how Observations synthesizes contemporaneous scientific, religious, and moral thought about human psychology. Then I relate Hartley’s views to subsequent psychological thought, identify parallels with concepts in past and present scientific psychology, and suggest the value of his synthesis for exploring the interface between psychology, and religion and spirituality. However, philosophical impediments in psychology’s traditions make such explorations unlikely without facilitative institutional changes.