1.1.1 Structure

It will help briefly to place the Summa theologiae, Aquinas’s most famous work, alongside some of his other writings. Aquinas’s Summa theologiae, or theological summary, (written from about 1265 to 1273) is his last of three major systematic theological works.Footnote ¹ The work is sometimes also referred to by the Latin title, Summa Theologica. The other two major systematic works are his Commentary on the Sentences of Peter Lombard (c.1252–1257) (Lombard lived c.1095/1100–1160) and the Summa contra Gentiles (c.1259–1265). Aquinas left the second (later) Summa unfinished at his death. He also wrote a number of Biblical commentaries on books from the Old and New Testaments. He also wrote a number of sermons that have been preserved. As a master at the University of Paris, Aquinas would conduct university-wide debates on certain topics with an audience of students and his fellow masters. This formalized procedure was known as a disputation (disputatio in Latin). (Disputations were also held in the religious houses of mendicant orders.) Aquinas would then arrange and edit the results of these debates for written publication. His series of Disputed Questions reflect when specific questions were set in advance of the dispute, such as in his works of questions On Truth (1256–1259), On Evil (c.1266–1270), or On Divine Power (c.1265–1266). His Quodlibetal Questions (c.1252–1256 and c.1268–1272) come from disputations where anyone in the audience could ask questions about any topic. The topics thus range widely. His considerable body of work also includes illuminating commentaries on a number of works of Aristotle and other thinkers.

The Summa theologiae is divided into three main parts.Footnote ² The First Part, or Prima Pars (abbreviated as Ia), focuses first on the nature of God, next the Holy Trinity, and then on creation. The discussion of creation includes treating the manner in which God creates and governs the world. The First Part also discusses the angels and the nature of the human person. The Second Part, which examines man’s journeying toward God, is divided into two main sections. The First Part of the Second Part, the Prima Secundae (Ia–IIae), covers human acts, morality, law, and the order of grace. The Second Part of the Second Part, the Secunda Secundae (IIa–IIae), includes a lengthy treatment of virtues as well as vices.

The Third Part of the Summa theologiae, or the Tertia Pars (IIIa), considers the Person of Christ, the sacraments, and the resurrection. However, after discussing Christ, Aquinas only got as far as the sacrament of baptism and a few questions on penance. After Aquinas left the work unfinished at his death in 1274, the Third Part was posthumously finished and is known as the Supplement. This Part continues on through the rest of the seven sacraments and treats the resurrection. The Supplement contains material on these topics gathered from Aquinas’s Commentary on the Sentences of Peter Lombard, the work likely having been done by Aquinas’s long-time secretary and friend, Reginald of Piperno (c.1230–1290). The three Parts of the Summa might be summed up as: Part One “on God,” Part Two “on man’s journey to God,” and Part Three “on Christ, the God-man, who is man’s road to God.”

Each Part of the Summa (First, Second, and Third) is broken down into three different levels of smaller units – (1) Treatises, (2) Questions, and (3) Articles. The articles themselves are divided into four stages. First, the three Parts of the Summa are divided into separate Treatises on general topics. For instance, in the First Part, there are Treatise(s) on Law, Man, Habits, Happiness, and Justice (among other Treatises). The Treatises are broken up into questions (quaestiones). Each question treats a major theme, such as God’s eternity (Ia q.10) or the nature of truth (Ia q.16).

Finally, each question is then divided into a series of articles covering distinct aspects of the main topic. Each article poses a question with a yes or no answer. Thus, question 2 of the First Part is on the existence of God. This question divides into three articles, which begin by posing a question: “Whether the existence of God is self-evident?” (a.1), “Whether God’s existence is demonstrable?” (a.2), and “Whether God [in fact] exists?” (a.3). The third article features the Five Ways.

The articles are structured as if they are conducting a debate. They do the detailed work of posing arguments for particular positions and working through objections. Each article poses an opening question with a yes or no answer and engages it in four sections: (1) objections, (2) a section known as on the contrary (sed contra in Latin), (3) a response (responsio), and (4) replies to objections. The four-step process reflects the process of a disputation. The first stage of an article poses a set of objections to how Aquinas will answer the opening question (usually in the affirmative). Second, the sed contra, or “on the contrary,” usually offers a brief, initial backing by an external authority for an opinion that Aquinas further develops in the response section. Here Aquinas often (but not always) cites an authoritative source such as the Bible, Saint Augustine (354–430), a Church Father, or even Aristotle. The third step is a response (responsio) where Aquinas develops his own arguments in response to the question posed by the article. The fourth part of the article replies to the original objections. Aquinas in this section takes stock of the original objections, usually in light of positions developed in the response section. The four sections of the article thus work together to settle its question.

Similarly, the different units of inquiry – Part, Treatise, Question, and Article – also work together in mutual support. The smaller articles work together to cover different angles of the larger question. The questions form the building blocks of the Treatise. In this way, the Summa organizes inquiry in a unified structure of different levels of increasing topical scope: Part, Treatise, Question, and Article, with each article formatted as a debate. All this gives the structure of the Summa an architectonic or architectural quality to it. The work has been compared to a cathedral of thought. The Parts and Treatises cohere the topics in the questions and articles into larger themes. The articles do the detailed intellectual work of defining terms, arguing positions, making distinctions, and resolving objections. They thus do the detailed work of sorting out various views according to where they stand with Aquinas’s own positions.

The articles have been likened to tiny pieces of a mosaic. The articles contain the particular arguments and positions of the Summa. But the fuller significance of these positions only emerges within the larger theological picture unfolding in the work’s Questions, Treatises, and Parts. This means that to see the real significance of a particular position, one must step back to take in the view of what surrounds it, in its relations with the other pieces making up the overall image. This makes it difficult and inadvisable to read a question or article as a self-contained and complete treatment of a problem. It is often crucial to consider the surrounding material (for instance, adjoining articles and questions) as well as topics Aquinas develops elsewhere in the work. It is why our careful look at each of the Five Ways will take us to many other places in both the Summa and Aquinas’s larger body of writings. Victor White observes:

(The same may be said of almost any philosophical or theological text. But readers of this enormously complex and detailed work do well to keep this in mind.)

Aquinas’s basic approach in the Summa is asking questions, airing different views, and then responding to objections. There is a focus on recognizing conflicts and resolving them by attempting to answer objections to one’s own position. The Summa thus reflects a significantly oral medieval intellectual culture where philosophers and theologians had to prepare to defend their views in large, public debates in front of crowds of students and colleagues.Footnote ⁴ One’s positions are strengthened and made more precise by facing opposition from other, different views. It is thus a work suffused with a strong sense of long-running interchange and debate. From all this, one hopes, a truer picture of the issues emerges.

1.1.2 Aims of the Summa

Why did Aquinas write the Summa? Likely for multiple reasons. His exact intents may never all be known for sure. His purposes may also have shifted during the years of composition. Fortunately, Aquinas leaves us clues. He states his purposes for the work at the beginning. Aquinas in the Prologue of the Summa theologiae explains that, “our intent in this work is to convey the things of the Christian religion in a way conducive to the education of beginners.”Footnote ⁵ He goes on to note the need to teach these things in an order suggested by the subject matter and in a way that avoids repetition and wordiness. There is an old joke about a student reading the Prologue and saying, “If this is the beginners’ manual, I’d hate to see the advanced version.” A work that is so difficult to read without guidance strikes few present-day readers as one for beginners.

The key is what one means by a beginner. Aquinas is not writing about Christian theology for absolute beginners, who have little or no preparation. Some scholars think that Aquinas likely has in mind beginning students of theology, who in his day would have had pre-requisite studies, particularly in philosophy, prior to starting sacred theology.Footnote ⁶ The Summa clearly expects readers to be familiar with concepts and positions then current in the field of philosophy. The Summa introduces the study of sacred theology by first asking, in the first article of the first question, whether any knowledge beyond philosophy is needed. The question would not be very intelligible except to an audience with some formal training in philosophy. Thus, the presumed level of intellectual training seems beyond, say, what a basic catechism for young adults today would expect. Leonard Boyle suggests a broader theory that the work might have started as an experiment by Aquinas to improve the theological education and formation of young friars, and possibly to improve the continuing education of, and as an advisory manual for, the general brethren already in ministry.Footnote ⁷ In any case, the work appears to have originated at least partly for teaching purposes. It seeks to offer a thorough, systematic treatment of Catholic Christianity.

Aquinas also gives some idea of the method of teaching. Aquinas proposes in the Prologue that the subject matter should be taught in a way apt for learning the discipline to help beginning students in the field. Aquinas contrasts his approach with other works where he says that the order of topics reflects an author’s need for commenting on books by other authors or as an opportunity for raising disputed questions. Aquinas aims to present the topics in a clear and concise way. He intends the discussion of topics to move from what is easier for students to understand and build up to more complex and difficult aspects of the subject matter.

Yet, long-time readers of the Summa grasp that the work is more than a teaching manual. Aquinas is not just summarizing Christian doctrine or repeating the day’s accepted positions. He develops his own complex views on high-level academic and Church-related controversies in his day. Aquinas offers positions and arguments which he has been refining for decades in his other writings. A commentator notes that Aquinas knows that other priests and scholars, including his intellectual and academic peers, will be interested in what he has to say on a variety of theological issues. Thus, the Summa goes beyond merely teaching students the received wisdom in the field. It offers highly sophisticated explanations defending his own philosophical and theological views. Aquinas wants his audience to learn about the Christian faith in order to know God, do God’s will, and then help others do the same. Its ultimate purpose is for its readers to live virtuously in order to achieve the final end of human life, to be with God in heaven.Footnote ⁸ The work can be read and well appreciated for its philosophical content. But it helps to keep in mind that the Summa theologiae is written primarily as a work of theology, not philosophy, and that its ultimate purpose is for its readers to seek eternal salvation for themselves and others.

Modern-day commentators often describe the topics of the Summa as flowing in a pattern known as exit and return (exitus-reditus). The idea is the following. The work begins with God in Himself, His existence, and nature. God is presented as both the source of all else that exists and that being which directs all other beings to their fundamental goals. Creatures flow from God as their source, the exit or going forth. Then each creature, in its own way, will return to God by way of providential design. The human soul does this by way of belief in God and the Trinity, and in Christ’s Incarnation, as well as by developing the virtues and participating in the sacraments of the Church. (All creatures are set to reflect the divine glory in some way.) Scripture affirms God as the Alpha and Omega, the beginning and end of all creatures in the visible and invisible realms.Footnote ⁹ This pattern of exit and return in the Summa is thought to mirror how theology as a discipline envisions reality, with God as the beginning and end of all things and as having a plan for the world’s redemption. (Summa theologiae Ia q.2 Prologue)

But the exitus-reditus structure of the Summa is a view questioned by some specialists of Aquinas. Its prominence as a theory only rose around the middle of the last century. The Summa contains topics that to some do not seem directly related to returning to God. There are long sections on material creation, the angels, ungraced human beings, the vices, and details on the structure of the Catholic Church. Brian Davies offers the measured view that it is not wrong but also not essential to read the Summa as having an exitus-reditus structure, “My point is that we can easily read it without thinking of Aquinas as having such a fixed schema in mind.”Footnote ¹⁰ It is a theory some may find interesting or helpful. Clearly an overarching theme of the work is creation issuing from God and the various means for bringing about the rational creature’s return to God. It is a theory worth knowing about. But how well it fits I will not pursue here.

1.2.1 Sacred Teaching in Q.1

Aquinas writes the Summa theologiae for believing Christians. Its first question (Ia q.1) is devoted to the nature and purpose of “sacred teaching” (sacra doctrina) in ten articles. Sacred teaching for Aquinas is about the divine revelation God gives to us, primarily in the Bible. Sacra doctrina is not limited to Scripture as a source. It includes what God reveals through His Word, or Christ, and also what is revealed in nature. Question 1 considers whether and why special teaching from God is needed. Perhaps philosophy or just common sense could suffice for guiding our views about the world, our conduct, and any sense we have of the divine and its purposes for us? Aquinas does not think so. There are some things about God, Aquinas thinks, which we can know using philosophy. But even this requires a lot of work and learning; it is a goal reachable by relatively few people. Even those people reputed to be wise, the influential sages of history, conflict with each other on divine matters and important questions of human purpose and conduct. (Contra Gentiles I c.4)

God’s revelation in Scripture also teaches us a great many things that we need to know for salvation, but which philosophy or everyday reasoning cannot arrive at. Other important truths that we need to know are only reasoned to with great difficulty and there is the likelihood of falling into error along the way. Even when philosophers arrive at some true positions, these are often mixed with much falsity as well. Human beings are created to have God as their final end or destiny. Getting there, Aquinas thinks, requires people to know things beyond the grasp of human reason. People also need a means of sorting out these important truths needed for salvation from the inevitable errors that occur when human thinking, left to its own devices, tries to approach the divine. Thus, Aquinas concludes that divine revelation is needed: “In order that the salvation of men might be brought about more suitably and surely, it was necessary that they be instructed in divine things through divine revelation.” (Summa theologiae Ia. q.1 a.1 resp.) Philosophy by itself is too fallible and insufficient to be our sole guide in the purpose and conduct of life.

This is why the Summa theologiae opens with q.1 on the nature and purpose of sacred teaching (sacra doctrina). Aquinas in this question also speaks about the discipline of philosophy and its relation to accepting Christian teaching on faith. Sacra doctrina is related to Christian doctrine (doctrina Christiana). The Latin word doctrina comes from the Latin word doctor or teacher. The latter comes from the perfect passive participle of the verb doceo, “to teach,” which is doctus. Doctrina conveys the sense of a body of knowledge, usually learned by being taught by someone proficient in that knowledge.

Sacra doctrina differs from how we normally think of the contemporary discipline of theology. Aquinas himself contrasts sacred teaching and theologia. For Aquinas, theologia has the sense of a philosophically derived metaphysics of the divine nature – the extension of metaphysics into what is now called natural theology. This would include ancient pagan views of the divine, derived by philosophy, such as those worked out by Plato (c.428–348 BC) and Aristotle. (Summa theologiae Ia q.1 a.7 sed contra) Instead, sacred teaching for Aquinas is expressly Christian in scope. It covers not just revelation in Scripture but an expanse of different meanings as Jean-Pierre Torrell explains:

Sacred teaching in its primary, initial sense is divine revelation from God.Footnote ¹² It is foremost instruction given to human beings by God. It is the divine revelation made known primarily through the Old and New Testaments, with a particular emphasis on the teachings of Jesus Christ in the Gospels. For Aquinas, God’s revelation, the theological truths needed for salvation, is to be passed on to the faithful by commissioned teachers.Footnote ¹³ These are instructors authorized by the Church to teach the Catholic faith, such as (in Aquinas’s time) a priest, bishop, a licensed preacher, or member of a university theology faculty. Aquinas was a master in Sacred Scripture (magister in sacra pagina), charged with the passing on of that revelation in Scripture by commenting on it and also by preaching sermons. The Dominicans, Aquinas’s religious order, is still known today as the Order of Preachers (Ordo Praedicatorum in Latin). Aquinas wrote many sermons which we still have today, but they are studied less often than his theological works such as the Summa theologiae and Summa contra Gentiles.

Aquinas holds that sacra doctrina has its proper grounding in the authority of canonical Scripture. (Ia q.1 a.8 ad 2) But he also recognizes sources involving what Catholicism now calls Tradition, the teachings of the Church, involving sources outside of Scripture. Sacra doctrina, he notes, thus looks to other authoritative sources such as the Doctors of the Church. He also regards as authoritative the Apostles’ Creed and the Nicene Creed. He recognizes (Summa theologiae IIae-IIae q.1 a.8 and 10) that the Church has the authority, in formulating creeds, to decide what teachings are to be held on faith by all its members. The pope can promulgate new editions of creeds to face the rise of heresies. Aquinas is also aware that there were believing Christians for a time well before the books of the New Testament were canonically put together. Aquinas, of course, is not familiar with the Scripture-Tradition debate since the Reformation, pitting some Protestants arguing for recourse to “Scripture alone” (sola Scriptura), against Roman Catholic and Orthodox Christians arguing for the value of Tradition in the Church’s teaching authority. We should be cautious about reading back into Aquinas some of the starkly defined views arising from that later controversy.

The important point for now is that Aquinas sees the teacher of sacra doctrina as reflecting on both Scripture and the teachings of the Church to render a reasonable and systematic account of what divine revelation is and what this revelation has to say to us.Footnote ¹⁴ Its core sources are the Bible, the Apostles’ Creed, and the Nicene Creed. Still, he thinks there can be discussion on how to interpret the meaning and implications of what these sources say.

1.2.2 Faith and Reason

Aquinas is well-known for holding that there can be no genuine conflict between authentic divine revelation (correctly interpreted) and the legitimate truths discovered by human reason.Footnote ¹⁵ Aquinas will speak of natural reason in a way in which we might think of as philosophy or just plain reason. He is considering what human beings thinking well can arrive at, without relying on what they take to be divine revelation. In a more specific way, Aquinas is thinking of how reason can demonstrate certain truths.Footnote ¹⁶ (I will say more presently about types of demonstrative arguments he thinks can be given.) Aquinas holds that conflicts between reason and the deliverances of faith are merely apparent. Such conflicts, he thinks, are at least in theory resolvable. Revelation has to be correctly interpreted and applied. Reason must be rightly used, according to what it can or cannot actually know about a subject. Aquinas thinks that it is crucial for people to respect the limits of what both reason and revelation can tell us about the particular matter under consideration. When people exercise due humility about what can actually be known for sure by reason or revelation, Aquinas thinks, then the findings of both are not found to be inconsistent. For instance, suppose someone claims that an argument grounded in philosophy or science refutes an element of authentic Christian revelation. In that case, either perhaps the argument has not well established its conclusion, at least to the degree of certitude needed, or possibly the revelation has not been accurately interpreted.Footnote ¹⁷

It is worth noting why Aquinas thinks reason and revelation do not at bottom conflict. First, he observes that reality cannot conflict with itself. If it is the case that Jim is now standing at the bus stop, it is false that he is not standing where he is. Objectively true statements cannot conflict with each other. Truth is one, that is, a single unified domain. According to Aristotle’s theory of truth, a statement is true when it accurately represents what is actually the case in the world. The theory is now called the correspondence theory of truth. Thoughts and expressions are true when what they depict corresponds with reality. What is proposed depicts reasonably well what is objectively the case. I can say, “Winston Churchill was born in 1874.” The objective facts surrounding his birth is what grounds or ratifies my utterance and thinking on the matter as true or false. For Aquinas, all truths have their source in God since God is the maker of all things. On this view, human reason when rightly used within its limits does not conflict with authentic revelation (rightly interpreted) that is from God, since God cannot be a source of untruth. God would not reveal things that are flat out contrary to what God is or intends, or contrary to the reality which God has made.

In affirming that faith and reason do not conflict, Aquinas is responding to differing views on faith and reason prominent in the schools at that time. A couple of these views I discuss later in this chapter, in connection with why Aquinas thinks philosophical arguments for God are needed. (Ia q.2 a.2) One prominent view of the time stems from the influence of Ibn Rushd (also called Averroes in the Latin West) (1126–1198), an Arabic philosopher well-regarded for his commentary on the thought of Aristotle. Aquinas in his own writings simply calls Averroes, “The Commentator,” without naming him. The difficulty arises when Averroes notes that Aristotle holds, for instance, that the world always existed and that human souls are not everlasting. Both positions are contrary to Christian doctrine, which affirms that the world has a (created) beginning and that human souls, once created by God, do not go out of existence.

Aquinas says that when philosophical positions go against doctrine, then the reasoning supporting them has to be flawed in some way or else is insufficient to prove the claim. Such positions, Aquinas thinks, turn out to be overly speculative. The arguments for these positions see the conclusions reach for a certitude which the premises cannot give. Aquinas thinks that it is the job of good philosophizing to show when and how this overreach occurs. When philosophical positions that are obviously contrary to Christian doctrine gain credence among an audience, he thinks that it is often because they lack the level of justification, or at least the surety, that some people think they have. Arguments made against the teachings of the Christian faith may seem derived from “first and self-evident principles embedded in nature.” But, Aquinas confidently holds, such conclusions against doctrine do not have “the force of demonstration [i.e., proof], but are either probabilistic or sophistical arguments.”Footnote ¹⁸ Thus, they can possibly be answered.

However, there were thinkers in Aquinas’s time who, taking inspiration from Averroes, developed a different solution to apparent conflicts between the findings of philosophy and Christian doctrine. This view starts from the idea that, when conclusions are reached that are contrary to Christian views, maybe the particular error in reasoning cannot readily be detected. Even earnest philosophizing may not be able to see what is wrong or settle the issue. Thus, as one view developed, controversial philosophical positions might be treated like conditional truths. They seem true by the standards of philosophy, as best one can see, if that is the measuring stick you are going to use. God, however, is a standard of absolute truth. His revelation presents truths beyond where secular philosophizing can reach. Faith and secular philosophy thus need to be compartmentalized from each other.

This sharp separation of the two sources produced what is called the “two truths” or “double truth” theory. Its basic idea is that philosophy and revealed theology constitute separate sources of knowledge that can lead to the acceptance of two contrary “truths.” The extent to which Averroes himself, or his followers in the medieval schools, actually held something like the existence of a “double truth” truth has long been debated. Some recent scholarship suggests that likely few followers of the movement divided the two sources of truth in a stark, unqualified sense. A prominent, long-standing view among scholars of medieval philosophy suggests that probably few followers (or none) held for a severe, unqualified (that is, equal and undiminished) acceptance of conflicting truths. Still, the view suggests a greater disconnect between reason and revelation than Aquinas is working with. In any case, Aquinas opposes the view. (Summa theologiae Ia q.1 a.1) The larger point here is to give one example of the way in which, in Aquinas’s time, faith and reason could be perceived as in tension or somehow ill-fitting with each other. This tendency toward separation would gain momentum in the decades after Aquinas’s death and into the fourteenth century.

This, for now, is a brief glance at some of the terrain through which Aquinas seeks to guide our thinking about faith and reason. He does not want faith and reason overly compartmentalized from each other, or seen as incompatible. He also does not want faith intertwined with reason, where Christian doctrine becomes increasingly subject to the standard of what philosophical reasoning will allow it to hold. Instruction from God is needed because human reason is limited, fallible, and (he thinks) easily swayed by pride or self-interest. Using philosophical reasoning as the final standard for accepting and interpreting revelation undermines for him the necessary supplementing and correcting of human knowledge that is the point of sacra doctrina.

Aquinas thus seeks a middle path. First, he explains that when it comes to truths about God, there are two types of truths (not an incompatible two truths), the 1) articles of faith and 2) the preambles of faith. The type called the articles of faith (articuli fidei) involves divinely revealed truths accepted on faith. They are not able to be demonstrated by reason. Where these truths are held, they are always held purely on faith. (Summa theologiae Ia q.1 a.1, IIa-IIae q.1 a.5) This category of beliefs includes the following examples: belief in the Trinity, the Incarnation, the doctrine of original sin, human atonement in Christ, and the bodily resurrection. What Aquinas considers to be articles of faith would be found in the teachings of the Apostles’ Creed and the Nicene Creed.Footnote ¹⁹ Reason can be employed to elucidate or deepen someone’s understanding of these truths. But it cannot prove them. For instance, philosophy might be used to explore metaphysical issues with a bodily resurrection; but belief in it is a matter of faith.

The second category of truths about God is called the preambles of faith (perambula fidei), or the preambles to the articles of faith. Think of a preamble as a preliminary statement introducing a document; one is ambling toward something. Aquinas thinks that philosophical argumentation can arrive at some truths about God. This includes establishing God’s existence and some basic attributes of the divine nature, in light of God being a first uncaused cause. These are prerequisites or matters which the articles of faith presuppose, as Aquinas notes in Summa theologiae Ia q.2 a.2 ad 1:

The preambles of faith involve certain positions, obtainable through philosophical reasoning, which the Christian faith presupposes. Lubor Velecky offers the example that the “logic of Christian faith requires all believers to think of God as really existing (though non-material),” as the uncaused cause of all else. Human reason can arrive at God as transcendent, immaterial, not subject to time, and not subject to certain ontological limits creatures are.Footnote ²⁰ It is not just that philosophical reasoning supports believing some things about God. There are also rival philosophical views not compatible with the perambula, such as that God is a material object or is confined to a certain physical place.

As the final sentence in the above passage from q.2 a.2 makes clear, Aquinas is not implying that each believer must philosophically work through these views before believing the Christian faith. In fact, he thinks relatively few people do, or the vast majority do little of this. If it were necessary to work them out for oneself, then most people would not arrive at them, Aquinas thinks. Few people have the combined ability, time, opportunity, and inclination to pursue the training needed to grasp metaphysical arguments about God’s existence and nature. Aquinas, as noted above, is also aware that the best philosophical minds of the ancient world disagreed among themselves on the nature of the divine. This of course does not preclude the same truth being believed on faith by some people and held for philosophical reasons by other people. For instance, Aquinas says in Summa theologiae IIa-IIae q.1 a.5 resp.: “It is possible for something that is seen or scientifically known by one person to be believed on faith by another. For instance, what we now believe on faith about the Trinity we hope one day to see.”Footnote ²¹ One person, for instance, may believe that God exists solely on faith, while someone else may assent to it based on reasoned arguments. Knowledge of the preambles does not mean that a person has to go further and accept the articles of faith, since the preambles do not rationally imply them.

Yet, while Aquinas says that the articles are accepted on faith, this does not mean the decision is a blind leap or arbitrary act of will, one wholly unsupportable by reason. Sacra doctrina rests on Biblical revelation, with particular emphasis on Christ’s teachings. Aquinas accepts the Gospels as reliable accounts of Christ’s teachings. He also accepts Christ to be divine because he understands Christ to have taught this about himself. Christ is said to be divine in the sense of being one subject with two natures, divine and human, as taught by the Church Council of Chalcedon. Brian Davies notes this emphasis on Christ’s teachings and sums up, “[Aquinas] thinks that the teachings of Jesus in the New Testament provide us with God’s final word on what God is and what God is doing.”Footnote ²² What Aquinas considers the truths of revelation in the Old and New Testaments are presumably mostly of the variety to be accepted on faith; they are not demonstrable by reason. However, in Summa contra Gentiles I c.6 Aquinas notes that people who assent to these truths do not do so in a light and foolish (non leviter) manner. Belief in Christian truths does not for Aquinas amount to a fideism, where belief is unconnected with reason or evidence, a blind leap of faith as it were. Revelation of the divine wisdom is, Aquinas thinks, accompanied “by fitting arguments” and with God giving “visible manifestation to works that surpass the ability of the whole of nature,” by which he seems to mean miracles confirming the divine origin of the revelation.Footnote ²³ In Summa theologiae IIIa q.43 a.4, Aquinas explicitly develops the point that Christ’s miracles manifest his divinity and testify to the veracity of his teaching. Christ’s claim to be God is confirmed by the miracles worked by divine power. Thus, while the articles of faith are accepted on faith, Aquinas thinks there are there are good reasons for our accepting them on this basis. The view here is of faith and reason as separate but complementary paths toward knowing some things about God.

Aquinas also is not committed to the position now known as evidentialism, that no belief should be held without sufficient evidence garnered by reason. Evidentialism holds that believing that God exists or not should be determined by whether I have enough objective evidence for that belief. On one such view, people should proportion their belief to the degree of evidence they have.Footnote ²⁴ Christian belief presupposes certain philosophical positions, as noted above. But, as explained above with the articles of faith, characteristically Christian beliefs are not philosophically derived from the preambles of faith. Moreover, many people in fact hold these preambles about God on faith. Mary might believe that there is a God because she has seen convincing arguments. John might just have faith that there is a God. Belief that there is a God, Aquinas says, does not have to be exclusively on one basis or another, either a successful metaphysical demonstration or a blind leap in the dark. Also notice that, on the question of whether reason can support certain truths about God or God’s plan for us, it depends on the content of the proposition one is talking about. Certain things about God are provable (the preambles), certain things are not (the articles). Aquinas is saying that one has to consider what is being believed and the person’s individual circumstances.

Sacra doctrina teaches that all of human experience refers back to God as the source and final end of persons’ lives. For Aquinas, this means that human reason finds its proper use and fulfillment only in a life cognizant of who God is and what God’s purposes for human beings are. Such cognizance, however, requires accepting on faith certain teachings of divine revelation. This is why the Summa begins with q.1 on sacra doctrina, outlining what Christian belief involves, and where the uses of philosophical reason come into the picture, or remain outside. For present purposes, this view of reason means that the arguments for God in q.2 are not just a kind of philosophical prelude to Christian theology. Aquinas is not just creating a platform of Greek philosophical theology on which he can stand a separate Biblical revealed theology. Victor White notes, “The Summa Theologiae is not, as sometimes supposed, a potpourri of theology and philosophy; it is wholly a Summa of Theology concerned with the Sacra Doctrina, the Holy Teaching of salvation given by God’s revelation. But because it is that, it can use philosophical argument to its own end – which is hominis salus – the health or salvation of man.”Footnote ²⁵

After Aquinas treats sacra doctrina in Summa theologiae Ia q.1, he argues for God’s existence in q.2 a.3, before articulating some basic attributes of God’s nature in qq.2–11. These attributes include God’s simplicity (q.3), supreme perfection (q.4) and goodness (q.5), infinity (q.7), omnipresence (q.8), immutability (q.9), eternity (q.10), and oneness or unicity (q.11). Treatment of the divine attributes is then followed by a discussion of human knowledge of God (and its limits) in q.12. Q.13 is devoted to how divine predication works, assigning different types of attributes to God’s nature. Thus, one overarching theme of these early questions of the Summa is that philosophy has its own power and integrity to treat matters having to do with God. Its role and reach are far from weak.

I also want to draw attention to Aquinas’s patient development of the attributes of God’s nature over a long series of questions and articles, which come after the Ways. As I explain in later chapters, the Ways do not give a full-blown concept of God. The subsequent questions after the Five Ways explicitly argue for the traditional attributes, often by appealing to the conclusions of the Ways. As laconically written, compact demonstrations, the five arguments are not designed then and there to respond to all kinds of inquiries and objections. That is why a careful explanation of the Ways must substantially draw from ideas and positions Aquinas speaks about elsewhere in his writings. Aquinas offers the Ways more in the spirit of getting the discussion about God going. At the same time, he does think of them as demonstrative “proofs” of God in a manner I will now explain.

1.3.1 Q.2 A.1 on Whether God Is Self-Evident?

Article 1 poses the question, “Whether the existence of God is self-evident (per se notum)?,” or more literally, “known through itself.” Here Aquinas is thinking of different ways a person might be thought to know God exists by a kind of direct or basic understanding of God we have. It is notably a route which does not use causal reasoning to argue for God as the cause of certain perceived facts about the world. Aquinas examines three ways God’s existence might appear to be “self-evident.” 1) Perhaps a knowledge of God is implanted in us by nature. We can tap an inherent sense that God is. 2) God’s existence logically follows once one understands a certain definition or defining concept of what God is. Analyzing a proper concept of God shows that God necessarily exists. 3) A third way identifies God with truth and then argues that God exists since truth evidently exists.

The first view is that persons have a kind of natural knowledge of God. This idea enjoyed wide acceptance in Aquinas’s day as well as in centuries prior to him. Article 1 quotes St. John of Damascus (c.676–749) from his An Exact Exposition of the Orthodox Faith, “the knowledge of God existing is naturally instilled in everyone (naturaliter est inserta).”Footnote ²⁷

Aquinas responds to this view first by noting that it is first a good idea to look into what a natural knowledge of God involves. All human beings naturally desire happiness and it is also true that God is our final end, “God is man’s beatitude.” Aquinas allows that “what is naturally desired by man must be naturally known by him.” Thus, a person naturally desires and so knows he or she wants happiness. Yet, Aquinas thinks, it is one thing to know this longing for happiness, which is thought to be fulfilled in this or that particular way. It is another matter to know, about the real object of our happiness, specifically that it is God and God in fact exists.Footnote ²⁸ By these standards, an atheist could have some vague desire to be happy, without realizing that the only true happiness is found in God. Aquinas illustrates this with an example. When a person approaches us from a distance, we may know that some person (in general) is approaching. But this does not mean that we know that it is Peter approaching. So too, many people confuse the true object of their happiness with pleasure, riches, and other temporal things. They believe there is an ideal state of fulfillment but lack specific, accurate knowledge of what it involves. Identifying our true happiness with God need not enter the picture or ensure that God is.

I will for the moment skip over the second view, that God’s existence is known by analyzing a concept. The third view of how God’s existence may seem self-evident involves a kind of argument from truth. (Ia q.2 a.1 obj.3) The basic idea is that God exists because God is truth and the truth obviously exists. Therefore, so must God exist. Augustine develops a long version of this argument in On Free Choice of the Will (De libero arbitrio c.3–16) and a shorter one in On True Religion (De vera religione 29, 52–30, 56). Aquinas notes in response, without mention of Augustine, that to acknowledge the existence of truth in general need not compel a particular belief in God. God is the transcendent standard and source of truth, but one need not acknowledge God’s existence in virtue of believing there is such a thing as truth. (q.2 a.1 ad 3)

The second way God’s existence might be seen as self-evident holds that God’s existence is logically entailed by analyzing a concept of what God is. This requires a longer discussion of the topic than the other two ways. Aquinas is speaking about a way of arguing for God that Saint Anselm of Canterbury (1033–1109) uses in chapters 2 and 3 of his Proslogion, known today as Anselm’s ontological argument. The idea is that once the term “God” is properly analyzed, then one can know on purely logical (non-empirical) grounds that such a being necessarily exists. As Aquinas explains: “For the name [God] signifies ‘that than which nothing greater can be signified.’ Moreover, that which exists in reality (in re) and in the understanding is greater than what exists in the understanding alone. Thus, since when the name ‘God’ is understood, He exists in the understanding, it follows that He also exists in reality.”Footnote ²⁹ As Aquinas presents the basic idea, to understand God in this way is inconsistent with our denying that God is real. Anselm’s version takes God to be something, “that than which nothing greater can be conceived [emphasis added]” (aliquid quo nihil maius cogitari possit).Footnote ³⁰ His original argument and defense is more elaborate than Aquinas’s basic summary of the argument. Aquinas does not directly cite Anselm, just as he does not cite Augustine’s argument from truth. Some recent commentators think that Aquinas’s main concern here is less with Anslem’s actual argument than with Anselmian-inspired theories held by some of Aquinas’s contemporaries. The idea is that the mind only has to turn inward in some way to know that God exists.Footnote ³¹ Whose thinking is brought to account here may be less important than Aquinas’s skepticism about any purely logical proof of God or inward-looking appeals to some mental process or intuition directly showing that God exists.

Aquinas in the sed contra of article 1, citing Aristotle, observes that it is not possible coherently to hold the opposite of a self-evident proposition. Yet even the Bible notes that “God exists” can be coherently denied. He cites the famous line from Psalm 53, “The fool has said in his heart, there is no God.” Many people deny that God exists and see nothing contradictory in their thought or statement of denial. Aquinas’s opening salvo here is saying that neither sound philosophy nor divine authority support God’s existence as self-evident to our minds – given the basic test that a self-evident proposition cannot be coherently denied. All human beings, including believers, can formulate the thought that maybe God does not exist.

However, in the response section of article 1, Aquinas does not altogether dismiss the idea that God’s existence is self-evident. It depends on what “self-evident” means. In the response Aquinas first notes that a proposition might be self-evident in two ways. A proposition may be 1) self-evident “in itself” (secundum se) or 2) it may or may not be evident relative to us (quoad nos), given our limited ability to know things. Aquinas defines a self-evident proposition in this context as one where “the predicate is included in the definition (ratio) of the subject.” Yet, what is evident in itself may not be self-evident to either every person or even any person at all. He offers the proposition “incorporeal things are not in a place” as one a wise person will see as self-evident. But not everyone will see this, he thinks, because it requires philosophically grasping the relation between being material and being in space.

Aquinas then goes to the second case, where something is self-evident in itself but no one can directly grasp that the proposition is self-evident, due to the limits of what the human mind can grasp. Interestingly, Aquinas thinks that “God exists” is self-evident in itself. It is of the very nature of God to exist. He explains here that “God is His own existence” (Deus enim est suum esse), the idea being that in God essence and existence are identical. Aquinas argues for this position later in the Summa, in Ia q.3 a.5, while treating God’s metaphysical simplicity. (I examine this intriguing identity claim in a later chapter.) Yet, “God exists” is not something self-evident to us. The human mind cannot know God in a manner that would make it self-evident to our mind. A finite created mind cannot comprehensively grasp the infinite divine essence to see this truth in a self-evident manner.

Aquinas near the end of article 2 turns to the Anselmian proposal that God’s existence can be self-evident to us. He views the situation as a matter of whether a person can grasp that “God exists” is self-evident in virtue of analyzing the significance of the term “God.” That is, he sees it come down to a question of whether the concept of God logically entails God’s existence. Aquinas has a couple thoughts on this in Ia q.2 a.2 ad 2. First, someone who hears the term “God” may have no idea of its Anselmian significance, that the term should be associated with the notion, “that than which nothing greater can be thought (cogitari).” For instance, someone might believe the divine nature to be something physical. Second, it is one thing to understand this signification, but it is another matter that someone directly understands the actual existence of this being. As Aquinas notes, it does not follow that what the person thinks the name signifies “exists in reality, rather than in intellectual apprehension alone.” It is one thing to know that God is the kind of being that, if God did exist, then God would always exist (never come to be or pass away). But this is no logical guarantee such a being actually does exist. The latter is “what is not granted by those who hold that God does not exist.”

Aquinas so far in q.2 a.1 has ruled out God’s existence being self-evident, he thinks, in a way we can grasp. He then poses the further question whether God’s existence can be made evident in some other way, free of the problems so far seen.

1.3.2 Two Types of Demonstration in A.2 of Q.2

Aquinas in article 2 considers two types of demonstration as ways of arguing for God. There is 1) what is called a demonstration propter quid and 2) a demonstration quia. Proper quid means “on account of which,” or as some commenters incline, “on account of the whatness (quiddity)” of a thing, where here the “whatness” or “what it is” of a thing is its essence, its fundamental identity. This type of demonstration moves from first grasping the essential definition or essential character of a thing and then arguing to certain features or facts about the thing such a grasp demonstrates. This type of demonstration can not only show that a certain feature is something a thing has. It also shows why it has the feature since there is a knowledge of the essence from which that feature flows. The human soul is immortal because, Aquinas thinks, it is by nature incorruptible. A stone does not jump up and run away from you because it is not a living animal with legs. These facts can be viewed, Aquinas thinks, as if they are like effects which are caused by the essence of what it is to be a human soul or a stone. Provided that you have a proper grasp of the essence, then you will intellectually see that essence entails having certain features. In any case, Aquinas holds that God’s existence is indemonstrable in this way.

There are a number of reasons for this, Aquinas thinks, only a few of which I will touch on here. (See Summa theologiae Ia q.12–13.) God’s essence is indefinable for the reason that God is infinite. God does not have a circumscribable essence limited to being an essence of a certain species kind, such as being a stone or a human soul or a zebra. The human mind is finite and cannot comprehend, that is fully grasp or reach around the nature of what God is. Our minds and any concepts we have are inherently finite. They are, Aquinas thinks, derived from creatures and particularly from sense experience of material objects. The human way of knowing is oriented toward grasping the essences of finite, material objects. It is not naturally suited in this life to knowledge of immaterial beings, much less an infinite one. With all this, human beings have no direct grasp of God’s essence. Aquinas thus holds that 1) God’s existence will have to be demonstrated, 2) by starting with certain facts about the world we can know, and reasoning from them to God as the first cause.

A demonstration quia (demonstratio quia) goes in the other direction. Here the reasoning goes from grasping something that is produced as an effect back to something that is a cause responsible for it. It involves demonstrating that a cause exists from the existence of its effects which are known to us: “For when an effect is more apparent to us than its cause, we proceed through the effect to a knowledge of the cause.”Footnote ³² The demonstration is effect-to-cause. It is called a demonstration quia (quia here means “that”) for the reason that the conclusion affirms the existence of the cause because the effect is known to exist. In this type of reasoning, it may only be possible to show that a cause exists, but not why it does. This demonstration also does not rely on a grasp of essences. Since God’s essence is neither directly nor comprehensively known, a demonstration propter quid will not work. But Aquinas notes (I q.2 a.2 ad 3), “from any effect the existence of the cause can be clearly demonstrated to us.” This does require stipulating a kind of nominal, working definition of what sort of cause one is looking for. (I will say more about this later in discussing the First Way.)

When Aquinas seeks to prove the existence of God in the Five Ways he will use a series of different nominal definitions for God. These will be rooted in the idea of God being the first cause of different observable effects in the world. His starting points will be the perceptions that in the world that things change (First Way), there is a certain order among causes leading to an effect (Second Way), that things undergo generation and corruption (Third Way), that things display gradations of being and perfection (Fourth Way), and that some things which lack intelligence show activity toward an end or goal (Fifth Way). The conclusions will designate a first cause accounting for beings having these effects.Footnote ³³ But notice these descriptions designate something of the effects of God as a cause; they do not spell out what the divine nature is in itself. They are taken from creatures’ relation to God (as their cause), not God’s essence.

Aquinas importantly takes the Five Ways to be demonstrations of God, where if the premises are true the conclusions must follow. These are not arguments that are merely probabilistic, that is, if the premises are true the conclusions are more likely true than not. Thus, he considers them demonstrative proofs of God. He introduces the Five Ways in Ia q.2 a.3 by saying that God’s existence can be proved (probari potest) in five ways. But seeing how Aquinas makes these five arguments first requires a sense of his metaphysics and views of causation. In the next two chapters, I now turn to this philosophical background essential to reading the Five Ways.