Hartley ,.I. David, an English philosopher, born in Armley, Yorkshire, Aug. 30, 1705, died in Bath, Aug. 25, 1757. He was educated at Jesus college, Cambridge, of which ho became a fellow, was destined to the church, but had scruples about subscribing the XXXIX. articles, and therefore studied medicine, which he practised with success at London, Bath, and other places. All records agree in extolling his personal character. His society was sought by the most distinguished literary men of his time. At the age of 25 he began the composition of his great work, "Observations on Man, his Frame, his Duty, and his Expectations," which was published after a labor of 18 years (2 vols., London, 1748-'9). His theory of sensation, grounded on an anatomical inspection of the nervous system, is historically curious as perhaps the first attempt to explain psychological phenomena on physiological principles. According to him, the white medullary substance of the brain, spinal marrow, and the nerves proceeding from them, is the immediate instrument of sensation and motion. External objects excite vibrations in the medullary cord, which are continued by a certain elastic ether.

Connected with this theory are other doctrines, especially that of association, which gave to Dr. Hartley a reputation as one of the most ingenious metaphysicians of the 18th century. When a sensation has been frequently experienced the vibratory movement from which it arises acquires a tendency to repeat itself spontaneously. Ideas are but these repetitions or relics of sensation, and in their turn recall other ideas. By the development of the law of association, and chiefly by the law of transference, he accounts for all the phenomena of the mental constitution. In many cases, the idea which is the link of association between two other ideas comes to be disregarded, though the association itself remains. Thus the idea of money is connected with that of pleasure by the conveniences which wealth may supply; but the miser takes delight in money without thinking of these conveniences. In this way Hartley accounts for almost all the human emotions and passions. An edition of the work, by his son, with notes from the German of H. A. Pistorius, was published in 1791 (3 vols., London). II. David, son of the preceding, born in 1729, died in Bath in 1813. As member of parliament for Kingston-upon-IIull, he steadily opposed the war with the American colonies.

He was one of the plenipotentiaries appointed to treat at Paris with Dr. Franklin, in whose correspondence, published in 1817, some of his letters appear. He was an early promoter of the abolition of the slave trade, and exhibited his scientific knowledge in several useful inventions.