More on David Hartley’s Association of Ideas:
What follows is a tour de force in which Hartley argues, case by case, that physical vibrations in the brain, spinal cord, and nerves are the basis of all sensations, all ideas, and all motions of men and animals and that all learning is the consequence of repetitive juxtapositions of corpuscular vibrations and mental associations in space and time, producing habits according to the pleasure-pain principle.
This principle, like that of association, has a long history, but in the new context of corpuscular physics and empiricist epistemology it took on a new significance.
The persistence of the pleasure-pain principle in biology, psychology, and the social sciences has allowed these disciplines to employ the physical analogies of the association of ideas without abandoning qualitative concepts based on the subjective world of experience.