Hartley David (1705–57), British physician and philosopher. Although the notion of association of ideas is ancient, he is generally regarded as the founder of associationism as a self-sufficient psychology. Despite similarities between his association psychology and Hume’s, Hartley developed his system independently, acknowledging only the writings of clergyman John Gay (1699– 1745). Hartley was one of many Enlightenment thinkers aspiring to be ‘Newtons of the mind,’ in Peter Gay’s phrase. In Hartley, this took the form of uniting association philosophy with physiology, a project later brought to fruition by Bain. His major work, Observations on Man (1749), pictured mental events and neural events as operating on parallel tracks in which neural events cause mental events. On the mental side, Hartley distinguished (like Hume) between sensation and idea. On the physiological side, Hartley adopted Newton’s conception of nervous transmission by vibrations of a fine granular substance within nerve-tubes. Vibrations within sensory nerves peripheral to the brain corresponded to the sensations they caused, while small vibrations in the brain, vibratiuncles, corresponded to ideas. Hartley proposed a single law of association, contiguity modified by frequency, which took two forms, one for the mental side and one for the neural: ideas, or vibratiuncles, occurring together regularly become associated. Hartley distinguished between simultaneous association, the link between ideas that occur at the same moment, and successive association, between ideas that closely succeed one another. Successive associations occur only in a forward direction; there are no backward associations, a thesis generating much controversy in the later experimental study of memory. See also ASSO- CIATIONIS. T.H.L.