The debt that Thomas Aquinas owes Aristotle is widely known. The extent of this debt is acknowledged perhaps most eloquently by Aquinas himself, who refers to Aristotle simply as "the Philosopher". Still, while Aristotle's philosophy provided much of his conceptual framework and vocabulary, Aquinas was not afraid to disagree with his master. An exegete far less subtle than Aquinas could realize that Aristotle and Paul did not always point in the same direction. When Aristotle's philosophy differed substantially from the Christian and Pauline tradition Aquinas was attempting to elaborate, Aquinas was forced either to reframe Aristotle's analysis, to extend it, or on rare occasions, to disagree with it outright. Aquinas indeed plundered Aristotle as the fleeing Israelites had spoiled the Egyptians , but Siger of Brabant was evidence enough that golden calves could be smelted from that same Aristotelian gold.
The degree to which Thomas was dependent on Aristotle, and the reasons for his occasional departure, can be seen clearly in their mutual accounts of the good life. With Aristotle, for instance, Aquinas agrees that the "good" is "that for the sake of which all else is done" (Comm. Nic. Eth., I, lect. 9), that happiness is the ultimate end of a human being (Compendium Theologiae, 106; Comm. Sent. lib. 3 d.27 q.2 a.2 co), and that happiness is an activity of the soul (Debated Questions, VIII, q.9 a.1; Sum. Theol. Iª-IIae q.3 a.2 s.c.). However, Aquinas differs from Aristotle on two important points, and in both of these instances, he departs from Aristotle for typically Pauline reasons.
First, for Aquinas, true happiness is not contemplation per se, but rather, contemplation of God in the beatific vision. As noted above, Aristotle's account of the good life is teleological, but not eschatological: he argues that we are happy when we are oriented towards the good, but he has no reason to believe that we will ever meet that good face to face. Following Paul, however, Aquinas believed that it was the destiny of creation not merely to travel hopefully, but actually to arrive. Aristotle perceived that there is something of divine origin in contemplation, but Aquinas goes further, saying that God is our true happiness, and that we may one day contemplate Him directly.
To be sure, God may not be known unless He directly enlightens the human intellect. Although we can only know the essence of an object through its species, we may know an object incompletely if we know a related genus: we might have never seen an ibex, but we can know something about it if we're told that it's like a deer. However, no creature has anything generically in common with God, so it is impossible for us to know His essence in this way. Aquinas' solution is elegant and incarnational, and turns Aristotle's epistemology on its head: "Therefore, so that God Himself can be known in His essence, it is necessary that God become the form of the knowing intellect, and join Himself to it" (Compendium Theologiae, 106). However, when God does so, this satisfies our natural desire for knowledge completely; this intellectual vision of God is thus the "end of our desire". "The act by which we are primarily united to Him is originally and essentially our happiness" (Quodlibetal Questions VIII, q.9 a.1 co). Or as Paul would have it, "Eye has not seen, nor ear heard, what God has in store for those who love Him" (1 Cor. 2:9; see Is. 64:4).
Second, Aquinas places love on an equal footing with knowledge in his account of happiness. Unlike Paul, Aquinas is careful to never deprecate the importance of scientia: he acknowledges, for instance, that we are primarily united to God "per actum intellectus" (Quodlibetal Questions VIII, q.9 a.1 co), and in that sense he is not far from Aristotle. However, while Aristotle can provide a nearly complete account of the good life without mentioning love , this would be impossible for anyone who regarded Paul's epistles as Scripture. Choosing Ephesians 3:19 as his proof-text ("supereminentem scientiae caritatem Christi"), Aquinas fundamentally relativizes the importance of the life of the intellect: "With respect to things that are above the soul, love (amor) is higher and nobler than knowledge; whereas in respect to those things that are below the soul, knowledge (cognitio) is more important" (Comm. Sent., lib.3 d.27 q.1 a.4 co).
Justitia is also an important part of Aquinas' perspective on the good life. In the Summa, Aquinas closely follows Aristotle's understanding of justice: like Aristotle, he defines justice as a "state of character" (Gr. ἕξις; Lat. habitus; IIª-IIae q.58 a.1 co), and hence a virtue (IIª-IIae q. 58 a. 2), which is concerned with equality between two parties (IIª-IIae q.58 a.2). Similarly, Aquinas divides justice into "general" and "particular" (IIª-IIae q.58 a.7), the latter consisting of "a certain proportion of equality between the external thing and the external person" (IIª-IIae q.58 a.10 co), and similarly divides particular justice into the two species of "justitia distributiva" and "justitia commutativa" (IIª-IIae q.61 a.1; Super Sent. lib. 4 d.17 q.1 a.1 qc.1 co).
However, beyond "general" and "particular", Aquinas introduces a third meaning of justitia, "a certain rectitude of order in the interior disposition of a man" ("rectitudinem quandam ordinis in ipsa interiori dispositione hominis", Iª-IIae q.113 a.1 co). He finds this definition tucked into Aristotle (NE V.13.1138b4), but while Aristotle makes little use of it in his Ethics, it's critical to Aquinas, allowing him to reconcile Aristotle with Paul's account of a God who "justifies the ungodly" (Rom. 4:5). Building on this definition, Aquinas argues that justificatio impii consists of a movement from internal disorder to right order. This movement comes entirely from God, though human free will cooperates: "He so infuses the gift of justifying grace that at the same time He moves the free will to accept the gift of grace" (Iª-IIae q. 113 a. 3 co). "God gives grace to none but to the worthy, not that they were previously worthy, but that by His grace He makes them worthy" (Iª-IIae q. 114 a. 5 ad 2). In this way, at least in theory, Aquinas maintains the Pauline order of receiving and then giving.
However, in practice, Aquinas' writings provide rather less opportunity for women and slaves than Paul allowed for. With Aristotle, he presents an extensive account of the inferiority of women, arguing that they are "deficiens et occasionatus" (Iª q.92 a.1). He similarly offers a basis for the institution of slavery as beneficial for the slave ("utile est huic quod regatur a sapientiori", IIª-IIae q.57 a.3 ad 2). Aquinas says that anyone who talks a slave into escaping is guilty of theft, because a slave is property (IIª-IIae q.61 a.3 co), and for this same reason, a slave cannot lawfully receive the sacrament of Orders (Supp. q.39 a.3).
Nevertheless, these exclusive tendencies are somewhat modified by statements which point toward a more inclusive perspective. For instance, Aquinas acknowledges that women are not naturally deficient with regard to general human nature (as opposed to their individual human nature, Iª q.92 a.1 ad 1). Similarly, he contends (IIª-IIae q.57 a.3 ad 2) that slavery belongs to "positive law" (jus positivum, laws originated by human beings), and not to "natural law" (jus naturale, laws originating in human nature). Consequently, because marriage is a matter of nature and not of human convention, slaves can marry without their masters' consent (Supp. q.52 a.2). Moreover, while the condition of slavery may affect the legality of the sacrament of Orders, it does not affect its efficacy (Supp. q.39 a.3 ad.5): divine grace is as available to slaves as to free. Aquinas does not perhaps make the same room for excluded classes that Paul does, but he clearly modifies Aristotle's doctrine of the "natural slave" in a more humane and inclusive direction.
The Beatific Vision
Aquinas follows Paul in asserting that divine love is our ultimate goal. In a rather startling passage, Aquinas argues for a nearly complete mutuality of love between God and human beings. Paul had exulted in the fact that we are "more than conquerors through Him who loved us" (Rom. 8:37), but Aquinas goes even further. Noting that Aristotle describes friendship as enjoyment of each other's company and a common pursuit of delightful activities (NE 1171b30-1172a5), Aquinas concludes that this may adequately describe not merely our love of God, but God's love for us: "It is therefore appropriate to acknowledge a certain friendship (amicitia) with God, by which we live together; and this is charity" (Comm. Sent., lib.3 d.27 q.2 a.2 co).
For Thomas, as for Paul, God is our end, and thus our happiness. In this beatific vision, both cognitio (as an act of the intellect) and amor (as an act of the will) are united. Because our wills desire God as their object, there is a sense in which the happiness of the beatific vision consists of our love for God. However, because this love is fulfilled only when our intellects actually perceive God, there is another sense in which the happiness of the beatific vision consists of our knowledge of God. Aristotle said that "pleasure perfects the activity" (NE 1174b20-1175a1), and Thomas uses this definition to merge these two conceptions: "Because this action [of perceiving God by the intellect] is most perfect and the object most worthy, the greatest joy follows, crowning this action and perfecting it, as beauty does youth" (Quodlibetal Questions VIII, q.9 a.1 co). The essence and source of happiness is thus in the intellect's vision, but the form and completion of happiness is in the will's joy and love.
Although Aristotle occasionally refers to God (or the gods), his moral philosophy is fundamentally secular in nature. The God who is the end of all things is nevertheless abstract and unknowable. If every reference to the divine were removed, Aristotle's ethics would for all practical purposes remain unchanged.
Paul would certainly have been sympathetic to some of Aristotle's positions, but the structure of Paul's gospel is fundamentally incompatible with Aristotle's secular perspective. For Paul, the good life has its beginning and its end in a God who loved His creation enough to become a part of it. As a result of the Incarnation and Resurrection, God's creation has changed absolutely and permanently. Paul's ethics are always and everywhere a response to God's action in human history and in our lives.
Aquinas plainly finds Aristotle's conceptual analysis helpful: he accepts much of Aristotle's ethical framework, borrows extensively from his vocabulary, and agrees with many of his conclusions, on occasion even when those conclusions stand in some tension with the New Testament. However, Aquinas is unable to accept any conception of happiness which does not have its origin and goal in God, and which is not finally expressed in love. As a result, he constructs a new framework around Aristotle's ethical theory by redefining happiness as a vision of God which completes itself in joyous love. With this one change, which has its origins in Paul's apocalyptic and inclusive theology of redemption, he is able to retain very nearly the rest of Aristotle's ethical theory.