Often the major contributions of self-effacing men are known best to those who come after them. Hartley's reputation as one of the founding fathers of the science of psychology rests on his two-volume work, Observations on Man, published, almost unnoticed, in 1749. Volume 2 deals with theology, and though of great importance to its author, it is of historical significance only as an example of the conviction held by many 18th-century scientists that there was no necessary conflict between science and revealed religion. Volume 1 is a systematic description of the emergence of highly complex emotional and mental states out of simple physical sensation.

Hartley's psychology can best be summarized under the twin headings of physiological determinism and associationism. All ideas and emotions are merely the coming together, or association, of various separate ideas which in turn can be traced to individual sensations that have been transmitted along the various nerves by a process of physical vibrations.

Hartley's private life was relatively uneventful: twice married, he fathered a number of children. Though he made little impression on the contemporary world at large, he communicated with many first-rate English minds of his day. In every way a gentleman, he was a kindly and expert doctor, well versed in subjects as diverse as shorthand and poetry and mathematics, devoutly religious, well organized and methodical though neither pedantic nor coldly efficient, and loved and admired by all who knew him. Hartley died at Newark, London, where he had practiced medicine, on Aug. 28, 1757.

The Transactions of the Halifax Antiquarian Society (1938-1939) contains a series of letters from Hartley to John Lister. These letters include personal information and show his interest in theological matters as well as algebra and shorthand. A facsimile of the first edition of Hartley's Observations on Man, edited by Theodore L. Huguelet (1966), contains a 12-page introduction which discusses Hartley's influence and life.

Hartley, David (1705-57) English physician and philosopher. Hartley is best remembered for being the founder of associationist psychology. His Observations of Man: his Frame, his Duty, and his Expectations (1749) is a naturalistic attempt to provide an integrated theory of human nature. Hartley believed that the task of education was to bring about an association of private pleasure with the exercise of public benevolence and virtue. In the well-adjusted person there is a ‘ladder of pleasures’ with those of benevolence towards the top, because of their associations with various other kinds of pleasure. Hartley's thought significantly influenced the later generation of utilitarians such as James Mill.

David Hartley (21 June 1705 – 28 August 1757) was an English philosopher and founder of the Associationist school of psychology.

David Hartley was born in June 1705 in the vicinity of Halifax, Yorkshire. His mother died three months after his birth. His father, an Anglican clergyman, died when David was only fifteen. He was educated at Bradford Grammar School and Jesus College, Cambridge, of which society he became a fellow in 1727. Originally intended for the Church, he was deterred from taking orders by certain scruples as to signing the Thirty-nine Articles, and took up the study of medicine. Nevertheless, he remained a member of the Church of England, and was on intimate terms with the most distinguished churchmen of his day. He considered it his duty to obey ecclesiastical as well as civil authorities. The doctrine to which he most strongly objected was that of eternal punishment.

Hartley was married twice. The first time in 1730 to Alice Rowley, who died the next year giving birth to their son David (1731-1813). His second marriage was in 1735 to Elizabeth (1713-78), the daughter of Robert Packer of Shellingford and Bucklebury, both in Berkshire. This marriage was undertaken in spite of the opposition of Elizabeth's influential and very wealthy family. This union produced two additional children, Mary (1736-1803) and Winchcombe Henry (1740-94). Hartley practised as a physician at Newark, Bury St Edmunds, London, and lastly at Bath, where he died in 1757.

His principal work Observations on Man, his Frame, his Duty, and his Expectations was published in 1749, three years after Condillac's Essai sur l'origine des connaissances humaines, in which similar theories were expounded. It is in two parts—the first dealing with the frame of the human body and mind, and their mutual connections and influences, the second with the duty and expectations of mankind. His two main theories are the doctrine of vibrations and the doctrine of associations. His physical theory, he tells us, was drawn from certain speculations as to nervous action which Isaac Newton had published in his Principia. His psychological theory was suggested by the Dissertation concerning the Fundamental Principles of Virtue or Morality, which was written by a clergyman named John Gay (1699—1745), and prefixed by Bishop Law to his translation of Archbishop King's Latin work on the Origin of Evil, its chief object being to show that sympathy and conscience are developments by means of association from the selfish feelings.

The outlines of Hartley's theory are as follows. Like John Locke, he asserted that, prior to sensation, the human mind is a blank. By a growth from simple sensations, those states of consciousness which appear most remote from sensation come into being. And the one law of growth of which Hartley took account was the law of contiguity, synchronous and successive. By this law he sought to explain, not only the phenomena of memory, which others had similarly explained before him, but also the phenomena of emotion, of reasoning, and of voluntary and involuntary action (see Association of Ideas). A friend, associate, and one of his chief advocates, was Joseph Priestley (1733-1804), the discoverer of oxygen. Priestley was one of the foremost scientists of his age.