Aristotle and Aquinas have been equivocated by many philosophers and theologians due to the symmetry of their respective philosophies. This treatment of these two philosophers does an injustice to their work as it forces Aristotle into a Thomistic shape and Aquinas into an Aristotelian shape; however, in both cases, the forms of their philosophies are two different to fit the respective shapes. In some cases the similarity may only be in the words that are used. Upon a deeper examination the appearance of substantial differences come to light that point to dissimilarities that make Aquinas and Aristotle less of a hyphenated philosophy and more like cobelligerents in similar streams of thinking.
Joseph Owens, in an essay in the Cambridge Companion to Aquinas, examines some of the substantial differences between the two philosophers that demonstrate the cleavages. In the examples provided by Owens, the differences are not obvious by a prima facie examination of the texts but require a systemic examination of the texts; that is, Aquinas and Aristotle seem to demonstrate symmetries because of the proximate outcomes of their philosophies, starting points and vocabularies. However, their differences lie both in their presuppositions and where they go with their philosophies as well as where they end up. In most cases, the differences lie in the fact that Aquinas had some place to go in continuing down a road that Aristotle saw no need to continue down precisely because of his Grecian theological presuppositions.
While there are a number of differences between the two philosophers noted by Owens, three examples are critical differences that change the geography of their respective philosophies to an extent that one would be unable to demonstrate any meaningful symmetries in them in order to put a hyphen between the two men’s names. First, Aristotle does not differentiate between being and essence where as for Aquinas this distinction is very real and quite necessary. Secondly, Aquinas specifically points to Aristotle’s thoughts on “the sempereternity of cosmic motion and time” and that his reasoning is contingent on these beliefs. And lastly, that being can not be belong to or be the nature of any other thing, contrary to Aristotle’s efficient causality.
In Aristotle’s Metaphysics, he comments that “Now although being and unity are a single nature in the sense that they are associated like principal and cause, they are not the same in the sense that they are expressed by a single concept.” In response to this, Aquinas responds, “Now the terms one and being do not signify different natures but a single nature. But things can be one in two ways; for some things are one which are associated as interchangeable things, like principal and cause; and some are interchangeable not only in the sense that they are one and the same numerically [or in the subject] but also in the sense that they are one and the same conceptually, like garment and clothing.” In this statement, Aquinas is taking a specifically Christian position that is critically at odds with the presuppositions of Aristotle concerning man. Aristotle sees no real reason to spend very much time concerning himself with the topic; however, this issue is critical for Aquinas in dealing with issues like the indestructibility of Human souls, animals, among other issues. For Thomistic thought then, this issue is pivotal to the Christian philosophical project. In Aquinas’ Commentary on the Sentences, he explicates this “In the thing there are both the quiddity of the thing and it’s being. So in the intellect there is a double activity corresponding to those two.” Those two activities are the “understanding of divisibles” as pointed out by Aristotle, and the other activity “comprehends the thing’s being.” As pointed out by Owens, “ [this is] the basis for the demonstration of a real distinction between God and creatures.” A distinction that was both denied by Aristotle and unneeded in this conception of Metaphysics. The criticality of this issue had, however, been dealt with before Aquinas’ treatment of Aristotle. Gregory of Nyssa recognized the problem of being in his anti-Gnostic tract “Against Eunomius,” it becomes clear through a reading of Gregory’s work that “[the Gnostics have] changed the ethical-physical of Christianity into the purely physical…banishing the spiritual and moral he has made his [unborn] as completely single and incommunicable…” It can be seen in Gregory’s tract, among other works and authors, that the problematic issues raised by an Aristotelian understanding of being go to the root of the Christian understanding of being and the soul. Thus, Aquinas not only disagrees with Aristotle, but also has basis for such a disagreement through the Church patrimony.
Aquinas was concerned with the Aristotelian tenet of the sempiternity of the universe. This doctrine is tied to other Aristotelian ideas that are contrary to the Christian philosophy as crafted by Aquinas. Aristotle contends, “There must be a principal of the kind whose substance is an actuality. Further, such substances must also be immaterial; for they must be eternal if anything else is. Hence they are actualities.” In response to this, Aquinas comments, “[Aristotle] concludes in this way last because of the question which he will next raise. From this reasoning, then, it is evident that here Aristotle firmly though and believed that motion must be eternal and also time; otherwise he would not have based his plan if investigating immaterial substances on this conviction…And aside from the other arguments which he does not touch upon here, it is evident that the argument which he does give here to prove that time is eternal is not demonstrative.” Aquinas also gives treatment to Aristotle’s thoughts on the sempiternity of the universe in other works. Else where in Aquinas’ commentary on Aristotle’s Metaphysics he also comments that “ Consequently, if the causes of motion proceed to infinity in this way, there will be no first cause. But a first cause is the cause of all things.” Aquinas does not attempt to argue the point philosophically, but demands that there is a first cause regardless of the rationalization of Aristotle otherwise.
The declaration by YAHWH, ergo sum qui sum, provides the basis for Aquinas’ theory of being. As summarized by Owens, “In philosophical language this meant that God was the first efficient cause of all other things. In this way, God was the primary instance of being. His was the nature to which all other beings had focal reference as beings.” Thus all creatures have contingent being on God’s efficient causality. This combines two necessary doctrines of God, first, that God is the efficient cause of creation, and second, that man derives his being from God and not himself. This will then become important to the idea of creation ex nihilo, as God is the efficient cause of matter and form, contrary to the Aristotelian ideal. Owens sums up this difference “So conceived, this is very different from the notion of being that had been developed by Aristotle. Yet it is readily brought under the general Aristotelian concept of actuality, which was adaptable enough to under go the further extension.” Aristotle fashions being as existing in man “seeing that one man and being a man and a man are the same…Moreover, the substance of each individual is…essentially a being…” Since each individual, composed of eternally existing matter, is “essentially a being” God plays no significant part for Aristotle in their existence. As seen above, this becomes radically different for Aquinas who, from a distinctly Christian world view, must account for a creation by God and a God who is intimately involved in the affairs of man, problems not dealt with by Aquinas.
To equivocate Aristotle and Aquinas seems thus to be a stretch of the imagination. While they have interesting symmetries of thought and agree on major points, for instance, on sensible objects and epistemology, it can be seen above that certain cleavages in their thinking keep them apart. Certainly to hyphenate Aristotle and Aquinas is to do the both a disservice and misrepresent their respective philosophies.
The necessity that Aquinas puts on being and essence differs from that of Aristotle. It is also critical to the understanding of the Christian concept of the soul. The sempiternally universe as opposed to ex nihilo creation demonstrates both the understanding of Aquinas of the place Christian thought on creation plays in philosophy and the critical difference between Aristotle’s thought and the Christian faith. Lastly, the Thomistic theory of being demonstrates the critical point at which the idea of creation ex nihilo and Thomistic thought on being and essence converge to demonstrate that in all three there are ideas posited by Aquinas that are contingent on one another, found in Aristotle, and contrary to Aristotelian thought. Owens summarizes this best “Yet each [Aristotle and Aquinas] of the two philosophies has to be kept carefully distinct from the other. Aristotle’s philosophy is based upon sensible natures, that of Aquinas upon sensible existents. To lump them together is to confuse their distinctive procedures and to deprive each of i
 Kretzmann, Norman , and Eleonore Stump, eds. The Cambridge Companion to Aquinas. New York: Cambridge University Press, 1993. Pgs 38-59.
 Ibid., 39,40.
 Aquinas, St. Thomas. Commentary on Aristotle’s Metaphysics. Notre Dame, Ind.: Dumb Ox Books, 1995. Pg 202.
 Ibid., 203.
 Kretzmann, Norman , and Eleonore Stump, eds. The Cambridge Companion to Aquinas. New York: Cambridge University Press, 1993. Pgs 39.
 Ibid., 58.
 Ibid., 39.
 Schaff, Phillip , and Henry Wace, eds. Nicene and Post-Nicene Fathers. Vol. 5. Peabody, Mass.: Hendrickson Publishers, 2004. Pg 50.
 Aquinas, St. Thomas. Commentary on Aristotle’s Metaphysics. Notre Dame, Ind.: Dumb Ox Books, 1995. Pg 791.
 Ibid,. 793.
 Ibid,. 114, 115.
 “I am who I am” (Ex 3:14)
 SCG I.22