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

David Hartley was born in 1705 in the vicinity of Halifax, Yorkshire. His mother died three months after his birth. His father, an Anglican clergyman, died when David was fifteen. Hartley was educated at Bradford Grammar School and in 1722 was admitted as a Sizar to Jesus College, Cambridge, where he was a Rustat scholar. He received his BA in 1726 and MA in 1729. In April 1730 he became the first layperson to be Master of Magnus Grammar School (Magnus Church of England Academy), Newark, and it was there that he began to practise medicine. On 21 April 1730, Hartley married Alice Rowley (1705–31). The couple moved to Bury St Edmunds, and Alice died there giving birth to their son David Hartley (the Younger) (1731–1813). While in Bury, Hartley met his second wife, Elizabeth Packer (1713–78), the fifth child and only daughter of Robert Packer (died 1731) and Mary Winchcombe, a wealthy and influential family with estates in Gloucestershire, Oxfordshire, and Berkshire, including Donnington Castle House, Shellingford Manor and Bucklebury House, Berks. (Mary Winchcombe was the daughter of Sir Henry Winchcombe, Bart., and the sister of Frances, wife of Henry St John, 1st Viscount Bolingbroke.) Despite the opposition of Elizabeth's family, David and Elizabeth wed on 25 August 1735, after agreeing to a severe set of restrictions that kept the £5,000 Elizabeth received upon her marriage completely out of the hands of her husband. Their first child, Mary (1736–1803), was born eleven months later. In 1736 the family moved to London, and then in 1742 to Bath, Somerset. When Elizabeth's last surviving elder brother died without issue in 1746, their son Winchcombe Henry (1740–94) inherited the family estates, making the family (though not Hartley himself) the possessors of significant wealth. Hartley died in Bath on 28 August 1757. He was buried at St John the Baptist Church, Old Sodbury, Gloucestershire.

At Cambridge, Hartley studied with Nicholas Saunderson, who, though blind since infancy, became the fourth Lucasian Professor of Mathematics. Hartley was later instrumental in raising the subscription for the posthumous publication of Saunderson's Elements of Algebra (1740). Upon graduation, Hartley declined to sign the Thirty-nine Articles, a requirement for ordination in the Church of England. Although one point at issue may have been the doctrine of the Trinity, Hartley's main dissent from Anglican orthodoxy was his assent to universal reconciliation. Writing to his friend Joseph Lister in 1736, Hartley stated he believed "[t]hat Universal Happiness is the Fundamental Doctrine both of Reason & Scripture", adding that "nothing is so irreconcilable [with] Reason as eternal Punishment, nothing so contrary to all the Intimations God has given us in his Works. Have you read Sr. Is. Newton’s Commt. upon Danl. & the Apocalypse?" For Hartley, on the gates of hell there could be no locks.

In the same letter to Lister, Hartley writes that "a Man who disregards himself, who entirely abandons Self-Interest & devotes his Labours to the Service of Mankind, or in that beautiful and expressive phrase of the Scriptures, who loves his neighbor as himself is sure to meet with private Happiness". This conviction became a guiding principle in Hartley's life, and it led him to devote himself to a various philanthropic projects. These include the publication of Saunderson's Elements of Algebra and the promotion of the shorthand system of his friend John Byrom (a system that Hartley believed could be a "universal character" and step toward the creation of a philosophical language).

Shortly after turning to medicine, Hartley became an advocate of variola inoculation for smallpox. Variolation confers personal immunity, and if widespread would be a "service to mankind" by furthering herd immunity. However, deliberate infection with the smallpox virus ran the risk of disfigurement or death. (Queen Caroline, wife of George II, was an advocate and had three of their children variolated, but Jonathan Edwards died from it in 1758.) The public good, then, could appear to be at odds with private interest. In his first publication, Some Reasons why the Practice of Inoculation ought to be Introduced into the Town of Bury at Present (1733), Hartley developed a statistical argument to show that the conflict is only apparent, that being inoculated furthers both the public good and a person's self-interest.

By the time of his move to London in 1736, Hartley was known by other campaigners of variolation, such as Hans Sloane and James Jurin, president of the Royal Society. He also had the support of important Whig families in Suffolk, notably of Charles Townshend, 2nd Viscount Townshend ('Turnip' Townshend). Hartley's daughter Mary wrote that "the old Lord Townshend (then Secretary of State) treated him with as much kindness as he had had been an additional son, and all the sons and daughters as an additional brother". He was inducted into the Royal Society, and he also became a physician to Thomas Pelham-Holles, 1st Duke of Newcastle, and his wife. In 1736 he offered to "recommend" John Byrom to George II.

By 1740, Hartley was known to every physician in London and to other medical men throughout Europe. He had thrown himself into a controversial attempt to harmonize private interest and public good. Hartley had started to experience symptoms of "the stone" (bladder stone) in early 1736. A bladder stone, sometimes as large as an egg, could function as a ball-cock on a toilet tank, causing an inability to urinate, excruciating pain, and sometimes death. (Benjamin Franklin, a sufferer, sometimes had to stand on his head to relieve himself.) Treatment by surgical removal (lithotomy) was a procedure many failed to survive.

Hartley thought a herbalist called Joanna Stephens had developed a lithontriptic, an oral medicine that would dissolve a stone in situ. He published Ten Cases of Persons who have Taken Mrs. Stephens’s Medicines for the Stone (1738), which includes an unsparing account of his own agonies. To make a proprietary medicine freely available to the public, Hartley convinced Parliament to pay Stephens £5,000 for her "secret".