How can Aquinas maintain that the heavenly bodies are eternal when they are material and all material things decay?
Thank you for a question which will be my pleasure to answer since the answer is one of the more interesting features of Aquinas’ cosmology.
Aquinas follows Aristotle in believing that heavenly bodies are not subject to decay because they are a different sort of material thing than other material things. Aquinas and Aristotle, first of all, believe that the heavenly bodies, the spheres, are eternal because it is a matter of observation that they move around the earth in uniform circular motion without any variation over many years. (Of course, closer observation has shown that these initial observations were wrong). Given that the motion of the spheres show no sign of slowing down, Aristotle and Aquinas thought there was no reason to suppose that they had ever sped up to their current speed or would slow down in the future. Thus, the eternity of the spheres applies primarily to their motion.
But if their motion is eternal, they must be eternal, and so Aristotle and Aquinas supposed that their forms totally fulfilled the potency of matter ( Summa contra Gentiles, Bk. II, Ch. 33). While everything in the world below the spheres (the sub-lunary world, as it is called), regardless of the form it has, still has matter which can turn into something else. This is the ontological, or natural basis, for decay. Something decays when something else comes to be from the matter of which the first thing is made. Since the spheres are eternal, they do not decay, and if they do not decay, their matter cannot turn into something else. Thus their forms completely actualize the potency of their matter. They are, in a sense, perfect material things. Since sublunary material things are made from the four elements (earth, air, fire and water) later thinkers said that the spheres were a fifth element or essence. Thus, they are the “quint (fifth) essence” of the material world. Hence, the origin of our word “quintessence” and “quintessential.”