The actual enunciation and discussion of principia by the Reformed orthodox, moreover, manifest both the reason for their treatment of the doctrine of predestination and the reason for its use as one focus, among others, of the theological system. Briefly, the Protestant scholastics declare two principia theologiae, a principle or foundation of knowing (principium cognoscendi) and a principle or foundation of being (principium essendi). The former is Scripture, the self-revelation of God, and the latter is God Himself, the self-existent ground of all finite existence. The first and foremost reason for the inclusion of any doctrine in such a system is the fact of its presence as a place or topic – a locus or topos – in the biblical revelation. Predestination receives considerable attention in the Reformed system. Its presence in the system, moreover, rests on the foundation of the Augustinian tradition antecedent to the Reformation.
More to the point, however, is the fact of the relationship of the doctrine of predestination to the doctrine of God. Alexander of Hales [1185-1245] and Thomas Aquinas [1225-1274], who have never been accused of creating a predestinarian system, noted the logically necessary relationship of the two doctrines by arguing whether predestination ought to be predicated of God. It is, after all, the eternal God who, in eternity, promulgates His decree concerning the shape and destiny of creation. This question of proper predication, incidentally, accounts for the occasional practice of Reformed scholastics of including the decree among the divine attributes. They were not attempting to create or to justify a deterministic system by the placement of a particular doctrine: they were simply reflecting a traditional question of predication.
From: Post-Reformation Reformed Dogmatics: The Rise and Development of Reformed Orthodoxy, ca. 1520 to ca. 1725: Volume One: Prolegomena to Theology by Richard A. Muller; 2nd edition (Grand Rapids: Baker Academic, 2003), pp. 126-127. The first edition was published in 1987.