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date: 19 October 2019


Abstract and Keywords

The book presents the contributions of Thomas Aquinas (1224/25–1274) as a theologist to society. The book covers his two most important works, Summa contra Gentiles (SCG) and Summa theologiae (ST) that presents the scope and philosophical character of medieval theology practiced by Aquinas. Several topics covered in those two large works are also investigated in more detail in the smaller works resulting from Aquinas's numerous academic disputations, which he conducted in his various academic posts. The book also explores Aquinas's most obvious philosophical connection with Aristotle. It also mentioned that Aquinas often adopts Aristotle's critical attitude toward theories associated with Plato, especially the account of ordinary substantial forms as separately existing entities. The book presents Aquinas as a paradigmatic Christian philosopher-theologian, who was fully aware of his intellectual debt to religious doctrine. He was convinced, however, that Christian thinkers should be ready to dispute rationally on any topic, especially theological issues, not only among themselves but also with non-Christians of all sorts. Aquinas differed from many of his thirteenth-century Christian colleagues in the breadth and depth of his respect for Islamic and Jewish philosopher-theologians, especially Avicenna and Maimonides. He saw them as valued coworkers in the vast project of philosophical theology, clarifying and supporting religious doctrine by philosophical analysis and argumentation.

Keywords: Thomas Aquinas, philosophical character, medieval theology, philosophical theology, Christian theology, Summa contra Gentiles

Thomas Aquinas (1224/25–1274) lived an active, demanding academic and ecclesiastical life that ended while he was only fifty (or a bit younger). He nonetheless produced many works, varying in length from a few pages to a few volumes. Because his writings grew out of his activities as a teacher in the Dominican Order and as a member of the theology faculty of the University of Paris, most are concerned with what he and his contemporaries thought of as theology. However, much of academic theology in the Middle Ages consisted in a rational investigation of the most fundamental aspects of reality in general and of human nature and behavior in particular. That vast domain obviously includes much of what is now considered to be philosophy and is reflected in the broad subject matter of Aquinas's theological writings.

The scope and philosophical character of medieval theology as practiced by Aquinas can be easily seen in his two most important works, Summa contra Gentiles (SCG) and Summa theologiae (ST). However, many of the hundreds of topics covered in those two large works are also investigated in more detail in the smaller works resulting from Aquinas's numerous academic disputations (something like a cross between formal debates and twentieth-century graduate seminars), which he conducted in his various academic posts. Some of those topics are taken up differently again in his commentaries on books of the Bible and/or works by Aristotle and other authors. Although Aquinas is generally remarkably consistent in his several discussions of the same topic, it is often helpful to examine parallel passages in his writings when fully assessing his views or the development of his views on any issue.

Aquinas's most obvious philosophical connection is with Aristotle. Besides producing commentaries on Aristotle's works, he often cites Aristotle in support of a thesis he is defending, even when commenting on Scripture. Although he dissents (p. 4) from Aristotle's views in many places, most notably those connected to ethics or metaphysics and theology, there are also in Aquinas's writings many implicit Aristotelian elements, which he had thoroughly absorbed into his own thought. He also often adopts Aristotle's critical attitude toward theories associated with Plato, especially the account of ordinary substantial forms as separately existing entities. Nonetheless, although Aquinas, like other medieval scholars of western Europe, had almost no access to Plato's works, he was influenced by the writings of Augustine and the pseudo-Dionysius. Through them he absorbed a good deal of Platonism as well.

On the other hand, Aquinas is the paradigmatic Christian philosopher-theologian, fully aware of his intellectual debt to religious doctrine. He was convinced, however, that Christian thinkers should be ready to dispute rationally on any topic, especially theological issues, not only among themselves but also with non-Christians of all sorts. Since in his view Jews accept the Old Testament and heretics the New Testament, he thought Christians could argue some issues with both groups on the basis of commonly accepted religious authority. However, because other non-Christians, “for instance, Mohammedans and pagans, do not agree with us about the authority of any scripture on the basis of which they can be convinced … it is necessary to have recourse to natural reason, to which everyone is compelled to assent—although where theological issues are concerned it cannot do the whole job” (since some of the data of theology are initially accessible only in Scripture).1 Moreover, Aquinas differed from many of his thirteenth-century Christian colleagues in the breadth and depth of his respect for Islamic and Jewish philosopher-theologians, especially Avicenna and Maimonides. He saw them as valued coworkers in the vast project of philosophical theology, clarifying and supporting religious doctrine by philosophical analysis and argumentation. His own commitment to that project involved him in contributing to almost all the areas of philosophy recognized since antiquity, omitting only natural philosophy (the precursor of natural science).

A line of thought with such strong connections to powerful antecedents might have resulted in no more than a pious amalgam. Aquinas's philosophy avoids eclecticism, however, because of his own innovative approach to organizing and reasoning about all the topics included under the overarching medieval conception of philosophical Christian theology, and because of his special talents for systematic synthesis and for identifying and skillfully defending, on almost every issue he considers, the most sensible available position.

Because Aquinas developed most of his thought within the formal confines of thirteenth-century theology, and because this has in turn affected his place in the history of philosophy and the assessment of his work, some attention must be paid to the ways in which much of what we recognize as philosophy was an essential component of what he thought of as theology.

Aquinas devotes the first three books of Summa contra Gentiles to a systematic development of natural theology, which he saw as part of philosophy.2 As part of philosophy, natural theology must be based entirely on “principles known by the (p. 5) natural light of intellect,”3 principles of the sort that underlie Aristotle's metaphysics, which Aristotle himself thought of as culminating in theology (see Aquinas's interpretation of that thought in the prooemium to his Sententia super Metaphysicam [Commentary on Aristotle's Metaphysics]). In fact, the way Aquinas works in SCG I–III strongly suggests that he may have thought of natural theology as a science subordinate to metaphysics, somewhat as he would have understood optics to be subordinate to geometry.

There is something odd about that project of his that scholars have sought to understand. By Aquinas's day, the churchmen governing universities had overcome most of their initial misgivings about the recently recovered works of the pagan Aristotle and had acknowledged officially that the study of Aristotelian physics and metaphysics was compatible with the then universally recognized availability of revealed truths about God. Medieval Christians had come to appreciate the ancient philosophers’ attempts to uncover truths about God on the basis of observation and reasoning alone as having been justified, even commendable, given their ignorance of revelation. Although no philosopher in Aquinas's circumstances could have justifiably undertaken a new project of natural theology heuristically, nonetheless, from their point of view no opprobrium would attach to natural theology taken up expositionally. The aim of such an enterprise would be not to develop theology from scratch but rather to show, in the spirit of Romans 1:20, the extent to which what had been supernaturally revealed could, in theory, have been naturally discovered. Such an enterprise is what SCG I–III seems to represent, in the view of some contemporary scholars.

What Aquinas himself says about his purpose in writing Summa contra Gentiles suggests that what he wrote had at least its formal cause in his consideration of the interrelation of philosophy and Christianity. He begins by writing about the concerns of a wise person, one of those “who give things an appropriate order and direction and govern them well.”4 Obviously, such a person has to be concerned with goals and sources, and so the wisest person will be “one whose attention is turned toward the universal goal, which is also the universal source,” which Aquinas takes to be God.5 Because this natural theology is oriented as it is, “it must be called the greatest wisdom itself, as considering the absolutely highest cause of all.”6 Therefore, the highest, most universal explanatory truth must be wisdom's concern.

Anyone aspiring to wisdom will attend to metaphysics, since, Aquinas reports, Aristotle rightly identified metaphysics as “the science of truth—not of just any truth, but of the truth that is the origin of all truth, the truth that pertains to the first principle of being for all things.”7 And, as Aquinas says in an observation that suits his own enterprise, “sometimes divine wisdom proceeds from human philosophy's starting points.”8 Nonetheless, since it is the business of one and the same science “to pursue one of two contraries and to repel the other … the role of the wise person is to meditate on the truth, especially the truth regarding the first principle, and to discuss it with others, but also to fight against the falsity that is its contrary.”9 The truth regarding the first principle will be the truth about God, supposing natural (p. 6) theology can show that God exists; and so the explanatory truth associated here with metaphysics is the truth associated also with theology.

In this pursuit by way of reason, Aquinas must and does shun “authoritative arguments” of any sort, but he shows good sense in not restricting himself to “demonstrative arguments” in developing natural theology. He does, of course, use demonstrative arguments when he thinks that he has them, but, like almost all philosophers of any period, he recognizes philosophy's need for “probable arguments” as well. A demonstrative argument takes as its premises propositions that explain the fact in the argument's conclusion by elucidating its causes (or, sometimes, its effects), and so it produces, or presents, scientific understanding. A probable argument, the sort that has always been most prevalent in philosophy, is one based on premises of any sort that are accepted widely or by experts in the relevant field; and so one group may be convinced by a probable argument that another group rejects.

In addition, Aquinas also frequently engages in what has come to be called philosophical theology, the application of reason to revelation. Philosophical theology shares the methods of natural theology broadly conceived—in other words, analysis and argumentation of all the sorts accepted in philosophy—but it lifts natural theology's restriction on premises, accepting as assumptions revealed propositions. This includes those that are initially inaccessible to unaided reason, such as the “mysteries” of Christian doctrine. In his many works of philosophical theology, Aquinas tests the coherence of doctrinal propositions (including the mysteries), attempts explanations of them, uncovers their logical connections with other doctrinal propositions, and so on, in order to bear out his conviction that the doctrines themselves are eminently understandable and acceptable, and that the apparent incoherence of some of them is only a feature of our initial, superficial view of them.

Aquinas's Summa theologiae is the paradigm of philosophical theology. The very first Article of the very first Question makes it clear at once that it is not natural theology of which Summa theologiae is a summa, since it begins by asking whether we need any “other teaching, besides philosophical studies,” which in Aquinas's usage means the studies that medieval beginners in theology would have just completed in the arts faculty. The question arises because philosophical studies are characterized not only as dealing with “the things that are subject to reason” but also as encompassing “all beings, including God,” as a consequence of which there is a part of philosophy that is theology.

Although Aquinas accepts this characterization of philosophy's subject matter as universal and as including a part that is properly called theology, he offers several arguments to support his claim that revealed theology is nonetheless not superfluous. In one of those arguments, he claims that a thing's “capacity for being cognized in various ways brings about a difference between sciences.” By this he means that different sciences can reason to some of the same conclusions on the basis of different premises or evidence. In his example, he points out that in order to support the proposition that the earth is round a naturalist uses empirical observations, while a cosmologist might support that same conclusion on a strictly formal basis. (p. 7) “And for that reason,” he concludes, “nothing prevents the same things from being treated by philosophical studies insofar as they can be cognized by the light of natural reason, and also by another science insofar as they are cognized by the light of divine revelation. That is why the theology that pertains to sacra doctrina [in other words, revealed theology] differs in kind from the theology that is considered a part of philosophy.”10

In this argument, Aquinas might appear willing to concede that revealed and natural theology differ only in this methodological respect, that they simply constitute two radically different ways of approaching the very same propositions about God and everything else. However, he would not actually concede this. There are propositions that belong uniquely to revealed theology's subject matter, simply because the different premises with which revealed theology begins can also lead to conclusions not available to unaided reason. And, of course, no doctrinal proposition that is initially available to human beings only in virtue of having been revealed by God can be part of natural theology's subject matter.

On the other hand, no propositions appropriate to natural theology are excluded from ST's subject matter. The propositions that belong to natural theology form a proper subset of those that belong to revealed theology:

It was necessary that human beings be instructed by divine revelation even as regards the things about God that human reason can explore. For the truth about God investigated by a few on the basis of reason [without relying on revelation] would emerge for people [only] after a long time and tainted with many mistakes. And yet all human well-being, which has to do with God, depends on the cognition of that truth. Therefore, it was necessary for human beings to be instructed about divine matters through divine revelation so that [the nature of human] well-being might emerge for people more conveniently and with greater certainty.11

When he sums up his examination of sacra doctrina, or revealed theology, Aquinas says that its “main aim … is to transmit a cognition of God, and not only as he is in himself, but also as he is the source of things and their goal, especially of the rational creature.”12 Thus, the subject matter of sacra doctrina, the theology presented in this summa of theology, is the most basic truths about everything, with two provisos: first, it is about God and about things other than God as they relate to God as their source and goal; second, among the things other than God with which it deals, it is especially about human beings, whose study of theology should be motivated by the fact that their well-being depends specially on their grasp of certain theological truths. And, Aquinas insists, universal scope is just what one should expect in a rational investigation of the truth about God: “All things are considered in sacra doctrina under the concept of God, either because they are God, or because they have an ordered relationship to God as to their source and goal. It follows from this that the subject of this science is really God,” even though the intended explanatory scope of the science is universal.13

In referring to sacra doctrina as a “science,” Aquinas means to characterize it as a systematic, reasoned presentation of an organized body of knowledge consisting (p. 8) of general truths about some reasonably unified subject matter. In that broadly Aristotelian sense, it is not obviously wrong to think of theology as a science (as it would be in the narrower, twentieth-century sense of “science”). It is in that sense that the science of theology as Aquinas develops it in ST would now be called philosophical theology, the enterprise of employing the techniques and devices of philosophy in clarifying, supporting, and extending the propositions that are supposed to have been revealed for theology's starting points. Thus, some of the work of philosophical theology is an attempt to explain revealed propositions and systematically work out their implications. Like natural theology, which is subordinate to metaphysics, philosophical theology is a subordinate science. However, because it begins its work on divinely revealed propositions, Aquinas identifies the science to which it is subordinate as God's knowledge of himself and everything else, available to human beings directly only in the afterlife.14 As he says earlier, “For us, the goal of faith is to arrive at an understanding of what we believe—[which is] as if a practitioner of a subordinate science were to acquire in addition the knowledge possessed by a practitioner of the higher science. In that case the things that were only believed before would come to be known, or understood.”15

Not even the doctrinal mysteries are impervious to rational investigation, although unaided reason could never have discovered them. Regarding one central mystery, for example, Aquinas says: “It is impossible to arrive at a cognition of the Trinity of the divine persons by means of natural reason.”16 However, he says this in the twenty-second of a series of seventy-seven Articles of ST devoted to analyzing and arguing about the details of Trinity, in other words, in the midst of subjecting this mystery to philosophical theology. As he explains in the very Article in which he rules out the possibility of rationally discovering that there are three divine persons:

There are two ways in which reason is employed regarding any matter … in one way to provide sufficient proof of something fundamental … in the other way to show that consequent effects are suited to something fundamental that has already been posited. … It is in the first way, then, that reason can be employed to prove that God is one, and things of that sort. But it is in the second way that reason is employed in a clarification of Trinity. For once Trinity has been posited, reasonings of that sort are suitable, although not so as to provide a sufficient proof of the Trinity of persons by those reasonings.17

Aquinas is also careful to point out that it is not mere intellectual curiosity or even a defense of the faith that is served by a rational clarification of the doctrine of the Trinity. In his view, this application of philosophical theology—confirming faith by reason, showing that belief in the Trinity is not after all irrational, exposing the intricate connections between these and other doctrinal propositions—aids one's understanding of creation and salvation.

The present volume is intended as a guide to Aquinas's thinking on almost all the major topics on which he wrote. In 1993 one of us (Eleonore Stump), together with Norman Kretzmann († 1998), edited a comparable volume, which appeared as The Cambridge Companion to Aquinas.18 That well-received volume, however, (p. 9) consisted of only ten essays, contained comparatively little on Aquinas's treatment of wholly theological issues, and had almost nothing to say about Aquinas's life and influence. The present book is much fuller. In addition to documenting Aquinas's life and work, it includes contributions that explain the Greek, patristic, Jewish, and Islamic influences on Aquinas's thought, and it also contains entries that show the historical reception and development of Aquinas's views. There are many more essays exploring the philosophical and theological topics discussed by Aquinas.

The book begins with a part devoted to historical background. This part includes an account of Aquinas's life and works by Jean-Pierre Torrell, whose Saint Thomas Aquinas: The Person and His Work is the currently most authoritative biography of Aquinas.19 It also contains a series of essays that set Aquinas in his intellectual context. These essays focus on the sources that are likely to have influenced his thinking, the most prominent of which (apart from the Bible, of course) were certain Greek philosophers (chiefly Aristotle), Latin Christian authors, such as Augustine, and Jewish and Islamic writers, such as Maimonides and Avicenna. The subsequent parts of the book address topics that Aquinas himself discussed. These include metaphysics, the existence and nature of God, ethics and action theory, providence and evil, epistemology, philosophy of mind and human nature, the nature of language, and an array of topics in philosophical theology, including Trinity, Incarnation, sacraments, and resurrection, among others. These parts include more than thirty contributions on topics central to Aquinas's own worldview. The final parts of the volume address the development of Aquinas's thought and its historical influence.

Although the volume thus aims at being comprehensive, readers familiar with Aquinas will undoubtedly find that some part of Aquinas's thought that strikes them as particularly important is not represented here. Sadly, it is not possible to do everything in one volume, even a fat one. At any rate, it is abundantly clear that some compromise is necessary between the ideal plan of presenting all of Aquinas's thought and any practicable plan for one book. We have tried to pick those issues and topics that allow a reader to see Aquinas's whole worldview in broad outline and to appropriate in particular some of its richest and most powerful parts.

Aquinas's philosophy includes reflection on some basic metaphysical topics while extending to discussions of what can be known by reason when it comes to the existence of God.20 He has views to offer on questions such as “What is it for something to exist?” “How should we distinguish between things in the world?” “What is it for something to be an individual in the world?” and “How should we understand causation?” He also has views to offer on questions such as “Can we know that God exists?” and “Can we give some account of God's nature?” Parts II and III of this book try to explain and comment on what Aquinas has to say by way of answer to such questions.

Yet Aquinas's intellectual interests range beyond metaphysics and natural theology. As the list of topics given above shows, he also had a concern with many other areas of philosophy as well. Aquinas often insists that moral philosophy would be redundant if people lack freedom of choice. So Part IV begins with an account of (p. 10) Aquinas's views on human freedom and agency, and it continues with consideration of Aquinas's approach to happiness, law, natural law, conscience, virtue and vice, and the theological virtues, among other things. Part V deals with Aquinas's theories of human knowledge and the nature of mind, as well as the relation of reason to faith. Part VI traces Aquinas's account of the nature of language and its limits when it comes to God.

The next part covers topics in philosophical theology. It includes an account of Aquinas's views on the problem of evil. Other chapters present Aquinas's account of the Trinity, the Incarnation, life after death, prayer, and the work of the Holy Spirit, among other things. Part VIII focuses on the progress of Aquinas's thought and its influence. It includes accounts of the ways in which Aquinas's ideas developed over time and the different ways in which subsequent thinkers have viewed and interpreted Aquinas's thought.

Finally, any attempt to present the views of a philosopher in an earlier historical period that is meant to foster reflection on that thinker's views needs to be both historically faithful and also philosophically engaged. So the present book combines both exposition and evaluation insofar as it is appropriate for any particular contributor to engage in both. It is our hope, therefore, that this Handbook will prove useful to someone wanting to learn about Aquinas's philosophy and theology while also looking for help in philosophical interaction with it.21

For invaluable assistance in preparing this volume for publication we are much indebted to Barb Manning, Stephen Chanderbhan, Zita Toth, and Gideon Jeffrey. We are also grateful to Peter Ohlin of Oxford University Press for helpful advice and for his patience in waiting for a work that was longer in the making than we originally expected it to be.


(1.) Summa contra Gentiles, I.2.11; hereinafter SCG.

(2.) Cf. Summa theologiae, I q.1 a.1 ad 2; hereinafter ST.

(3.) ST I q.1 a.2.

(4.) SCG I.1.2.

(5.) SCG I.1.3.

(6.) SCG II.4.874.

(7.) SCG I.1.5.

(8.) SCG II.4.875.

(9.) SCG I.1.6.

(10.) ST I q.1 a.1 ad 2.

(11.) ST I q.1 a.1.

(12.) ST I q.2, intro.

(13.) ST I q.1 a.7.

(14.) ST I q.1 a.2.

(15.) Expositio super librum Boethii De trinitate 2.2, ad 7.

(16.) ST I q.32 a.1.

(17.) ST I q.32 a.1.

(18.) Cambridge: Cambridge University Press.

(19.) Jean-Pierre Torrell, Saint Thomas Aquinas: The Person and His Work (Washington, D.C.: Catholic University of America Press, 1996).

(20.) It is often said that Aquinas thinks that he can prove the existence of God. But he does not. For Aquinas, God's existence is identical with his essence, which Aquinas takes to be incomprehensible to us. He does, however, argue that we might make a philosophical case for “God exists” being true (on a certain understanding of “God”). For the distinction between “the existence of God” and “God exists,” see Lubor Velecky, Aquinas’ Five Arguments in the “Summa Theologiae” 1a 2, 3 (Kampen: Pharos, 1994).

(21.) Parts of this introduction are revised versions of sections in the entry “Thomas Aquinas,” by Norman Kretzmann and Eleonore Stump, Encyclopedia of Philosophy (London: Routledge Press, 1998) and a small section of the prefatory material taken from Eleonore Stump's Aquinas (London and New York: Routledge, 2003).