Born Aug. 30, 1705, in Armley; died Aug. 28, 1757, at Bath. English thinker; one of the founders of associationist psychology.

A clergyman’s son, Hartley studied theology at Cambridge. Later on he received a medical education and practiced medicine all his life.

Striving to establish the precise laws of mental processes for controlling human behavior, he sought to apply the principles of Newtonian physics for that purpose. According to Hartley, vibrations of the outer ether cause corresponding vibrations in the sense organs, brain, and muscles, and the latter are in a relationship of parallelism to the order and connection of mental phenomena, from elementary sensations to thought and will. Pursuing the theory of J. Locke, Hartley for the first time made of the mechanism of association the universal principle for explaining mental activity. He considered repetition to be fundamental for the reinforcement of association. As Hartley saw it, the mental world of the individual takes shape gradually as a result of the complication of primary elements through association of mental phenomena by virtue of their contiguity in time and frequency of repetition; the motive forces of development are pleasure and pain. Hartley in like fashion explained the formation of general conceptions; these arose out of individual conceptions through the gradual disappearance of everything fortuitous and unessential from an association, which remains immutable; all of its constant features are retained as an integral whole thanks to speech, which acts as the factor of generalization.

Though mechanistic, Hartley’s theory was a major step forward on the way to a materialistic interpretation of the mind. He influenced not only psychology but ethics, aesthetics, logic, pedagogy, and biology as well. The English chemist and philosopher J. Priestley vigorously propagated Hartley’s teachings.