During his years of study in Edinburgh, he became involved with the Rankenian Club, a circle of intellectuals, which included those who would become his closest associates: George Young, the surgeon and William Wishart, a preacher.
At this time, Turnbull was interested in creating a rational form of Christianity, which led to his correspondence with John Toland, and the writing of an essay in defence of religious toleration which was never published.
These two works indicate that he was the first Scottish thinker to publish writings that argued for the use of the so-called Newtonian method in constructing a moral philosophy.
At this point, Turnbull decided that he might seek employment in the Anglican church, and matriculated at Exeter College, Oxford, in 1733 and received a BCL degree.
He published a small tract in 1731 which was inspired by a passage in Lord Shaftesbury's writings: A philosophical enquiry concerning the connexion betwixt the doctrines and miracles of Jesus Christ, where he maintains that just as experiments confirm scientific theories, so the miracles of Jesus Christ confirm Christian doctrine.
Turnbull's last significant work was published in 1742: Observations upon Liberal Education, where he suggested a new categorisation and breakdown of knowledge, and a revision of university curriculum.