Etienne Gilson on Medieval Realism contra Greek Realism

Medieval realism thus became the heir to Greek realism for quite another motive than that which inspired the philosophy of Aristotle; and it is this that gives it its own peculiar character. Aristotle turned away from Platonic idealism because man's kingdom is a kingdom of this world, and because above all else we need to know something of the world in which our lot is cast. Christians turned away more and more resolutely from Platonic idealism because the kingdom of God is not of this world, but because the world on the other hand, is necessary as a starting point from which to rise to the kingdom of God. To dissolve it into a flux of inconsistent appearances is to snatch from us our best means of rising to the knowledge of God. If the work of creation were not intelligible what could we ever know of its Author? Were we presented with nought but an Heraclitean flux, would a work of creation be even imaginable? It is ust because all is number, weight and measure that nature proclaims the wisdom of God. It is precisely in its fecundity that it attests to His creative power. Because things are of being, and no mere quasi-nought, we know that He is Being. Thus what we learn concerning God from revelation the face of the universe confirms: "The creatures of this visible world signify the invisible attributes of God, because God is the source, model and end of every creeature, and because every effect points to its cause, every image to its model, every road to its goal." Suppress all knowledge of the effect, the image, and the road, and we shall know nothing of the cause, the model, and the goal. The philosophical realism of the Middle Ages was nourished on Christian motives, and a realism there will always be as long as the influence of Christianity continues to make itself felt.

Etienne Gilson, The Spirit of Medieval Philosophy p. 244.