In 1754 Bourn moved to Norwich to assist the presbyterian minister John Taylor, who three years later left for Warrington Academy.
He was born at Crook near Kendal, and educated at Stand Grammar School and the University of Glasgow where he studied under Francis Hutcheson and John Simson.
In 1752 the publication of his first sermon under the title The Rise, Progress, Corruption and Declension of the Christian Religion, led to overtures from the presbyterian congregation at Norwich, and in 1754, apparently after the death of the senior minister, Peter Finch, Bourn became the colleague of John Taylor.
When Bourn came to them they were worshipping in St Mary the Less, Norwich, an ancient edifice, then held by trustees for the Walloon or Huguenot Protestants.
He was no optimist; he devoted a powerful discourse to the theme that no great improvement in the moral state of mankind is practicable by any means whatsoever (vol.
John Mason joined the debate conducted by published sermons in a two volume work called Christian Morals.
[1] Bourn published in 1764 a rejoinder encapsulated presbyterian doctrine appended to an earlier work known as Discourses on the Parables of our Saviour.
During his career Bourn moved to a more Arian christology in the philosophical mould of Samuel Clarke, rejecting the trinity doctrine and justification by faith, rationalising Christ's deification as the Son of Man.
Samuel Parr, headmaster of Norwich grammar school took him to Cambridge, and spoke of him as a masterly writer, a profound thinker, and an intimate friend.