[2] In the preface to the Defence, Crombie states that personal freedom was discussed in his divinity course under Beattie, who followed Thomas Reid and the commonsense philosophy, and supported libertarian free will.

He changed his view in the direction of necessitarianism after reading Joseph Priestley's Doctrine of Philosophical Necessity Illustrated (1777).

He was spurred into print by reasoning from James Gregory's Philosophical and Literary Essays (1792), and his 1793 work draws heavily on Priestley's arguments.

[12][13] An obituary of Crombie was written by John Grant A.M (c.1770–1846), "philologist and critic" of Crouch End, who had published a Latin grammar in 1808.

[1][18][19] It stated that in early life, Crombie was acquainted with Joseph Priestley, Richard Price and Alexander Geddes.

[24][25] Crombie was married at least twice: By his first marriage, he had a son: He remarried, on 6 March 1798, at Cluny, Aberdeenshire, to Jane Nory (c. 1773–1859); the couple had children including: