proved to be perfectly agreeable to the PRINCIPLES OF MORAL PHILOSOPHY.” And Christian Philosophy does indeed seek to do exactly what the advertisement declares. It is therefore probable that, whatever the publisher’s motive for inserting the composite title in some copies of volume two, the two volumes were, according to the author’s intention, a single book.
In light of the advertisement it might be speculated (without being strongly urged) that the discrepancy in title pages may be related to Turnbull’s poor health; perhaps at the time of going to press with volume one the publisher was uncertain whether he would ever see the second volume. The nature of his illness is unclear. In a letter to Thomas Birch dated 7 April 1739, Turnbull had said he had a bad cough and that this was a new illness; and again on 4 February 1740 to Birch, he described himself as “seriously ill.” It might be conjectured that he was suffering from bronchitis or tuberculosis, but there is at present insufficient evidence. In the earlier of the two letters he comments that he is revising “a work which has long lain by me called the Moral philosopher …” and adds that he plans to revise the work that summer.6 If his publisher was aware of these medical details, as seems probable, he might well have been doubtful of his prospects of getting the second volume and consequently did not call the first “volume one”; only be latedly could he publicly acknowledge the unity of the work by adding the composite title.
In the preface to volume one Turnbull declares that aside from “a few things taken from late writers” the work is the substance of several pneumatological discourses that he had read more than twelve years earlier to students of moral philosophy, and he adds that the lectures were delivered at the time of publication of his two “theses,” that is, the public orations he delivered in 1723 and 1726 on the occasion of the graduation of his first and second cohorts of students.7 It is almost certain, therefore, that the young Thomas Reid heard the lecture-room version of Turnbull’s Principles of Moral Philosophy, and this points to Turnbull’s place in the early stages of the Scottish school of common sense philosophy. But quite aside from the probability that Turnbull had a major influence on Reid, The Principles of Moral and Christian Philosophy is of great interest in itself for the doctrines it develops.
Turnbull indicates what he himself regards as his true intellectual context by mentioning some of those who have influenced him. He singles out John Clarke’s Boyle lectures, Bishop Berkeley (mainly the Treatise Concerning the Principles of Human Knowledge), Lord Shaftesbury’s Characteristicks, Bishop Butler’s Analogy of Religion, Alexander Pope’s An Essay on Man (a work which had just been defended, to Turnbull’s delight, by William Warburton, a theologian of whom Turnbull strongly approved), and Francis Hutcheson—“one whom I think not inferior to any modern writer on morals in accuracy and perspicuity, but rather superior to almost all” (p. 14). While, on the basis of Turnbull’s own words, he is sometimes said to be particularly indebted to Hutcheson, it should be noted that Turnbull’s earliest publication, the graduation oration of 1723, predates Hutcheson’s earliest publication by two years and shows Turnbull already well set on the course he was to pursue for the rest of his writing days. The probability is that Turnbull and Hutcheson, educated in the same philosophical-theological canon and relying otherwise on their own native genius, reached rather similar conclusions without either having a great influence on the other.
Shaftesbury may have been a much greater influence on Turnbull than Hutcheson was. Shaftesbury’s importance is indicated by Turnbull’s early membership in the Rankenian Club (founded 1716 or 1717), an Edinburgh society composed mostly of young men preparing for the church or the law, who were particularly interested in Shaftesbury’s ideas and wished to create a forum to discuss them. Shaftesbury’s writings were also the focus of attention of the circle of thinkers who gathered round Lord Molesworth in Dublin, and it is therefore of interest that Turnbull, though never a member of the circle, maintained a correspondence with Molesworth on the subject of the relation between liberty, education, and the need to raise standards in the universities.
On the highly informative title page of the Principles of Moral Philosophy, Turnbull quotes Sir Isaac Newton’s Opticks book 3: “And if natural philosophy, in all its parts, by pursuing this method, shall at length be perfected, the bounds of moral philosophy will also be enlarged.” In a way this says it all. Turnbull places moral philosophy not outside but within natural philosophy. Natural philosophy is an empirical study of nature pursued by the method of observation and experiment, and for Turnbull, as for Newton, human minds, which are the proper object of the study of moral philosophy, are parts of nature. In that case the moral philosopher should rely on observation and experiment as his principal means of discovering the powers, affections, and operations of the mind. By such means the laws governing the human mind will be laid bare.
Natural philosophy is an investigation into the laws governing the behavior of things in the natural world. The laws are discovered via a search for uniformities in behavior. But Turnbull believes it is possible not only to discover the laws of nature but also to demonstrate their inseparability from a set of values, for the laws play a part in the production of the goodness, beauty, and perfection of the natural world. And the crucial point for Turnbull is that this is true whether we are speaking of the corporeal world or the moral world, that is, the world of spirits, human and otherwise. The principal objective of volume 1 of the Principles, therefore, is the identification of the laws of human nature and the demonstration that they serve the good, both the individual good and the good of the whole moral system. Insofar as the laws of nature, so to say, deliver a world that is good and beautiful, and insofar also as the laws are not themselves beings with intellect and will capable of intentionally delivering such a world, they have to be seen as pointing to a divinity beyond the natural world that they structure, a being who does have intellect and will, and who has a providential care for the world he created. The laws are therefore God’s instruments created to form a world that measures up as well as any world could to his goodness.
From Turnbull’s perspective, indeed, from that of almost every theologian of the Western tradition, the goodness of the world is a very imperfect representation of God’s goodness. But though imperfect, it is the best possible for a created world, and it is for this reason that Turnbull repeatedly refers to the world’s “perfection.” Furthermore, though always aware of the limits of our intellectual powers as we seek insight into the mind of God, Turnbull thinks that progress in this quest is possible because we can make discoveries regarding the natural world and especially regarding the laws of nature as statements of God’s intentions for this world. In this sense Turnbull’s thinking in the Principles is in line with that of his colleague and friend at Marischal College, the mathematician Colin Maclaurin,8 as well as that of a number of other leading contemporary scientists, who believed that recent scientific discoveries, and particularly those of Newton, constituted the best possible evidence for the existence and the attributes of God. Turnbull holds that in this sense natural science spills over into natural theology, or rather natural theology is one of the facets of natural science, just as—so Turnbull indicates at the start of the Principles—natural science spills over into moral philosophy, or rather moral philosophy is one of the facets of natural science. For Turnbull, therefore, the three apparently disparate disciplines constitute a strong unity.
The first law Turnbull identifies is “the law of our power,” by which the existence or nonexistence of certain things depends on our will, and here Turnbull refers to the existence, or otherwise, of things whether in our minds only or in the outer world. For by an act of will we produce physical artifacts and we also have ideas—it is a matter of great importance to Turnbull that thoughts are no less subject to our will than are the movements of our limbs. In this sense we have “dominion,” though limited, in the corporeal world and the spiritual. Such dominion is a kind of liberty. With dominion over my limbs I am at liberty to move them, and when I exercise that dominion my limbs move not of their own accord but by my determination. Now there is a view that liberty and law are incompatible, for law encroaches upon and thereby constrains the scope of liberty. But Turnbull rejects this and argues, to the contrary, that it is only in a world governed by natural laws that
