Thomas Aquinas on Celestial Matter

The Thomist 68 (2004): 431-67 THOMAS AQUINAS ON CELESTIAL MATTER STEVEN BALDNER St. Francis Xavier University Antigonish, Nova Scotia, Canada IT HAS BEENFORTYYEARS since the publication of Thomas Litt's magisterial study ofThomasAquinas's doctrine ofthe heavenly bodies.1 That work provides a comprehensive and accurate presentation of the Thomistic understanding of the heavens: their nature, motion, causality, place, and purpose in the universe. It remains the indispensable scholarly source for understanding Thomas's teaching on these topics. Nevertheless, it has established one tenet of the Thomistic position that I wish to challenge. According to Litt,2 and most other commentators as well,3 Thomas Aquinas holds that the heavenly bodies are incorruptible because they are composed of a prime matter that is different from the prime matter of earthly bodies. The problem arises as follows. Thomas as a good Aristotelian understands that all material substances are composed ofform and 1 Thomas Litt, Les corps celestesdans l'univers de saintThomas d'Aquin (Louvain & Paris: Publications Universitaires, 1963). 2 Ibid., 6-7. 3 JohnWippel,TheMetaphysicalThoughtofGodfrey ofFontaines (Washington, D.C.: The Catholic University ofAmerica Press, 1981) 286-87; Edward Grant, Planets, Stars, and Orbs: The Medieval Cosmos, 1200-1687 (Cambridge: Cambridge University Press, 1996), 250-51; Michel-Pierre Lehmer, Le monde des spheres I: Genese et triomphe d'une representation cosmique (Paris: LesBellesLettres, 1996), 142-43;Joseph Bobik,AquinasonMatterandForm and the Elements: A Translation and Interpretation ofthe De Principiis Naturae and the De Mixtione Elementorurn ofSt. ThomasAquinas (Notre Dame: University ofNotre Dame Press, 1998}, 199-205. On the other hand, Robert Pasnau, Thomas Aquinas on Human Nature (Cambridge, Cambridge University Press, 2002), 137, claims that there cannot be two kinds of prime matter in Thomas's doctrine. 431 432 STEVEN BALDNER matter. By virtue of one and only one substantial form, any substance is what it actually is. This means that whatever is actually true about the substance, just insofar as it is substance, is caused by the substantial form. On the other hand, by virtue of matter in the most basic sense, which is prime matter, any substance is liable to substantial change; that is, it is liable to become a completely new actual thing. This means that both generation and corruption are possible because substances are not merely actual; they are potentially other, and they are so because of prime matter. To put this another way, it is because of prime matter, and only because of prime matter, that a substance is subject to substantial corruption. But if this is so, then it seems that one can claim that a material substance such as a heavenly body is incorruptible only if one says that it does not have prime matter in the sense in which other material substances have prime matter. That is, if the heavenly bodies have a different sort of prime matter, which has an appetite for only one substantial form, then one could explain the incorruptibility of the heavenly bodies by asserting that the prime matter of heavenly bodies is different in kind from the prime matter of earthly bodies. This is just the position attributed to Thomas Aquinas by Litt and by those who have followed Litt's interpretation of Thomas. I intend to show, however, that Thomas's position was not always, and was not finally, what has just been described. Thomas, in fact, altered his position in important ways more than once. Under the influence of his teacher, St. Albert the Great, Thomas in his early works held an Averroistic position, that the heavenly bodies are not composed of form and matter. In his maturity, when he wrote the Prima pars of the Summa Theologiae, Thomas did hold the position attributed to him by Litt. In his last works, however, such as his commentary on the De caelo of Aristotle, Thomas seems to have adopted a third position, namely, that the heavenly bodies are incorruptible by virtue of form, not by virtue of matter. To show this, I shall discuss three topics: the doctrine of Averroes and Albert on the incorruptibility of the heavens, AQUINAS ON CELESTIAL MATIER 433 Thomas's early position in the 1250s, and Thomas's mature position in the 1260s...