Francis Turretin's Institutes of Elenctic Theology was THE text used to train generations of Presbyterian and Reformed pastors from the late 17th through the mid-19th centuries. He is a prime example of what Richard Muller has categorized as the "high orthodoxy" period (ca. 1640-1685-1725) of Protestant scholasticism (see Post-Reformation Reformed Dogmatics vol. 1 Prolegomena to Theology). Last year, as I studied the doctrine of the Sabbath, a friend recommended him. Turretin was most helpful, giving me precise categories with which to properly understand the biblical doctrine of the fourth commandment. Just the other day another friend encouraged me to read Turretin on the law of God. I began last night. Turretin's biblical insight and well-reasoned precision are wonderful. Here's what he has to say about the difference between the natural law and the decalogue:
If it is asked how this natural law agrees with or differs from the moral law, the answer is easy. It agrees as to substance and with regard to principles, but differs as to accidents and with regard to conclusions. The same duties (both toward God and toward our neighbor) prescribed by the moral law are also contained in the natural law. The difference is with regard to the mode of delivery. In the moral law, these duties are clearly, distinctly and fully declared; while in the natural law they are obscurely and imperfectly declared both because many intimations have been lost and obliterated by sin and because it has been variously corrupted by the vanity and wickedness of men (Rom. 1:20-22). Not to mention other differences: as that the natural law was engraven upon the hearts of men, the moral on stony tables; the former pertains to all universally, the latter only to those called by the word; the former contains nothing except morality, the latter has also certain ceremonials mingled in it.Hence is easily gathered the reason why God wished to recall that law by Moses, to deliver it to his people viva voce, and proclaimed it in a solemn manner, committing it to writing and comprehending it in the decalogue. For although in upright nature there was no need of such promulgation, still (after sin) so great was the blindness of mind, such the perversity of will and disturbance of the affections that only remains of this law survived in the hearts of all (like rubbed pictures of the same, which on that account ought to be retouched by the voice and hand of God as by a new brush) (11.1.22-23a).
I'm sad to report that I've not read much Turretin. But I plan to remedy that soon.