Early life and education (1733–55)
Priestley was born to an established English Dissenting family (i.e. they did not conform to the Church of England) in Birstall, near Batley in the West Riding of Yorkshire. He was the oldest of six children born to Mary Swift and Jonas Priestley, a finisher of cloth. To ease his mother's burdens, Priestley was sent to live with his grandfather around the age of one. He returned home, five years later, after his mother died. When his father remarried in 1741, Priestley went to live with his aunt and uncle, the wealthy and childless Sarah and John Keighley, 3 miles (4.8 km) from Fieldhead. Because Priestley was precocious—at the age of four he could flawlessly recite all 107 questions and answers of the Westminster Shorter Catechism—his aunt sought the best education for the boy, intending him for the ministry. During his youth, Priestley attended local schools where he learned Greek, Latin, and Hebrew.
Around 1749, Priestley became seriously ill and believed he was dying. Raised as a devout Calvinist, he believed a conversion experience was necessary for salvation, but doubted he had had one. This emotional distress eventually led him to question his theological upbringing, causing him to reject election and to accept universal salvation. As a result, the elders of his home church, the Independent Upper Chapel of Heckmondwike, refused him admission as a full member.
Priestley's illness left him with a permanent stutter and he gave up any thoughts of entering the ministry at that time. In preparation for joining a relative in trade in Lisbon, he studied French, Italian, and German in addition to Aramaic, and Arabic. He was tutored by the Reverend George Haggerstone, who first introduced him to higher mathematics, natural philosophy, logic, and metaphysics through the works of Isaac Watts, Willem 's Gravesande, and John Locke.
Priestley eventually decided to return to his theological studies and, in 1752, matriculated at Daventry, a Dissenting academy. Because he had already read widely, Priestley was allowed to skip the first two years of coursework. He continued his intense study; this, together with the liberal atmosphere of the school, shifted his theology further leftward and he became a Rational Dissenter. Abhorring dogma and religious mysticism, Rational Dissenters emphasised the rational analysis of the natural world and the Bible.
Priestley later wrote that the book that influenced him the most, save the Bible, was David Hartley's Observations on Man (1749). Hartley's psychological, philosophical, and theological treatise postulated a material theory of mind. Hartley aimed to construct a Christian philosophy in which both religious and moral "facts" could be scientifically proven, a goal that would occupy Priestley for his entire life. In his third year at Daventry, Priestley committed himself to the ministry, which he described as "the noblest of all professions".