The ancient philosophers did not attribute that which is best and highest to the First Principle, because they considered only the imperfect material principle. God is Perfect, because He is the First Efficient Principle, supreme in Actuality, and, therefore, supremely Perfect. In Him are the perfections of all things, for whatever perfection exists in the effect must be found in the efficient cause, and thus they exist in God in a more eminent manner than is the case in creatures. For God is His own very Existence of Himself, and hence it must be that He contains all perfection of being, for perfection is identified with being. The creature is like to God, because God is the Efficient Cause of all, and every agent does a work like to itself in proportion to its actuality. If an agent is one in species with its effect, there is likeness between them in species, as man generates a man; and if they are not in one species, there is likeness, but not in species, as those things which are generated by the sun's heat are like to the sun in some degree, but they do not receive the form of the sun in specific likeness, but only in generic likeness. If there is an Agent outside of Genus, the effect has a more remote likeness to it; for the likeness is not based either on genus or species, but only on analogy, inasmuch as both have being. In this way creatures are like to God, the First Natural Principle of all.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni