Abstract
The paper is concerned with Aquinas´s doctrine of natural law set forth in the "Tractate on Law" of his Summa Theologiae. The author shows that Aquinas´s doctrine of natural law is an original synthesis between Augustine´s concept of the eternal and natural law and some elements of Aristotle´s ethics and of Aristotelian-Neoplatonic metaphysics of Arabic provenance (especially Avicenna).