In 1732, he received the honorary degree of doctor of laws from the University of Edinburgh.He seems to have mingled in the literary circles of London,' and acquired friends among persons of eminence.During these years he prepared an immense store of literary works, which were issued in rapid succession, -- more rapidly, I suspect, than the public were prepared to receive them.In {98} October, 1739, he advertises, at four guineas, in sheets, his " Treatise on Ancient Painting;" in which he has observations on the rise, progress, and decline of that art among the Greeks and Romans, comments on the genius of Raphael, Michael Angelo, Nicholas Poussin, and others, and illustrates the work will)engravings of fifty pieces of ancient painting.It will be remembered that Shaftesbury had " Disquisitions on Taste;"and we shall see most of the Scottish metaphysicians speculate on taste and beauty.The work was not of such an original or daring character as to recommend it to the genius of Hogarth; yet it seems to have had a considerable roll of subscribers." is style is pleasant, and the remarks judicious and highly appreciative of the classical painters.
In February, 1740, there appeared his most important work, and the only one that continues to be read, "The Principles of Moral Philosophy." At the close he promises, as soon as his health admits, a work on " Christian Philosophy," which was actually published before the close of the year; and in it he treats of the Christian doctrines concerning God, providence, virtue, and a future state, and recommends the Word of God because it embraces and illustrates such doctrines.He dates October, 1740, a preface and appendix to Heineccius's " Methodical System of Universal Law." In 1742, he published " Observations upon Liberal Education;and in it he speaks as having long been engaged in the work of education.He subscribes himself as Chaplain to the Prince of Wales, and dedicates the treatise to the " Right Reverend Father in God," Thomas, Lord Bishop of Derry; in whose esteem, he says, " he had long had a share." It appears that before {99}this time be had left the communion of the Presbyterian Church of Scotland, and entered into orders in the Episcopal Church of England, which was doubtless more congenial to his tastes.Through the bishop, to whom he dedicated his work, he was appointed Rector of Drumachose, in the diocese of Derry.I cannot find that he left any mark behind him in that parish: there is no remembrance of him in the popular tradition of the district, and no record of him in the diocese.In consequence of failing health, he went to the Continent, and died at the Hague, Jan.31, 1748.
Turnbull was the first metaphysician of the Scottish --I believe of any -- school to announce unambiguously and categorically that we ought to proceed in the method of induction in investigating the human mind.He takes as the motto of his " Moral Philosophy " the passage from Newton about the method of natural philosophy being applicable to moral subjects, and the line of Pope, " Account for moral as for natural things." His enunciations on this subject are as clear and decided as those of Reid and Stewart in after ages."If a fact be certain, there is no reasoning against it; but every reasoning, however specious it may be, -- or rather however subtle and confounding, -- if it be repugnant to fact, must be sophistical." It must have been from Turnbull that Reid learned, even as it was from Reid that Stewart learned, to appeal to common language as built on fact or universal feeling."Language not being invented by philosophers, but contrived to express common sentiments or what every one perceives, we may be morally sure that where universally all languages make a distinction there is really in nature a difference." Reid only catches the spirit of his old master, who speaks of " philosophers who, seeking the knowledge of human nature not from experience, but from Iknow not what subtle theories of their own invention, depart from common language, and therefore are {100} not understood by others, and sadly perplex and involve them selves." In some respects, his exposition of the method is more comprehensive and correct (so I believe) than that given by Reid and Stewart; inasmuch as he avows distinctly that, having got facts and ideas from experience, we may reason deductively from them, in what Mr.J.S.Mill calls the deductive method, but which is in fact a joint inductive and deductive method.He sees clearly that in natural philosophy there is a mixture of experiments with reasonings from experiments; and he asserts that reasonings from experiments may have the same relation to moral philosophy that mathematical truths have to natural philosophy." In both cases equally, as soon as certain powers or laws of nature are inferred from experience, we may consider them, reason about them, and compare them with other properties, powers, or laws." He instances among the moral ideas which we may compare, and from which we may draw deductions, those of intelligence, volition, affection, and habit.Moral philosophy is described by him as a mixed science of observations, and reasoning from principles known by experience to take place in or to belong, to human nature.
In his preface to Heineccius, he says that the appended "discourse upon the nature and origin of laws is an attempt to introduce the experimental way of reasoning into morals, or to deduce human duties from internal principles and dispositions in the human mind." In following this method, he claims to be superior to Puffendorf, to Grotius, and the older jurists.
