Thomas Aquinas came from a powerful medieval family related to the Hohenstaufen Holy Roman emperors.i Despite his powerful position, Aquinas chose to devote his life not to power or wealth but to God, becoming a priest in the religious order of St. Dominic. The Dominicans were a new order in the Catholic Church, and they and the Franciscans had started a reform movement characterized by active outreach and voluntary poverty that rebuked the atmosphere of religious complacency and decadence which they perceived in the parts of the Church.ii During his priestly training, Aquinas was influenced by the philosophical interests of Albertus Magnus, and he began to write philosophical and theological works of his own.
Aquinas is still studied in philosophy classes for his brilliant synthesis of Christianity and the philosophy of Aristotle. The prevailing mode of Christian thought before Aquinas was Platonism, due to the influence of Augustine of Hippo, an early Christian theologian who had been a Platonist prior to his conversion. He, along with several other early thinkers, accomplished a synthesis of Plato and Christianity, adapting Platonic language and concepts for Christian thought.iii In the tradition of Augustine, Aquinas resolved to accomplish a new synthesis of Christianity and ancient philosophy, which he believed could convey the Christian religion to his contemporaries more effectively and precisely than the predominant Platonic model.
Until shortly before Aquinas’ time, Aristotle was largely unknown in the West, though his works were preserved in the East, especially in Alexandria, by Jewish and Muslim scholars. It was through contact with these thinkers that Aristotle became known in the West.ivThis fact made some Christians doubly dubious towards Aristotle, as he both challenged Plato and came mediated through Jewish and Muslim sources. However, Aquinas, encouraged by Albertus Magnus, embraced Aristotle. He became a prominent defender of Aristotle and incorporated Aristotle’s thought and language into his philosophy and theology.
Aquinas felt comfortable undertaking such incorporation because, as he said, “All truth is one.”v He argued that what we learn from the natural world through science and philosophy, provided it is unquestionably true, can never contradict that which we learn from revelation, that is, directly from God.vi He compared Scripture and reason to two books, “the book of revelation” and “the book of nature,” which were both “written” by God and consequently compatible. Therefore, though Aquinas was well educated in the Bible and the writings of earlier theologians, he preferred to base his arguments in logic and philosophical reasoning that could appeal even to nonbelievers.vii He did so confident in his faith that reason and philosophy would confirm and not contradict the revelation of God.
One of the most important shifts in Christian thought associated with Aristotle's influence on Aquinas is an affirmation of the concrete, physical reality of the human condition.viii Like Plato himself, Christian Platonists tended to emphasize the subjective and the abstract, the realm of purely spiritual forms and ideas. This philosophical emphasis could translate into a felt contempt for matter and the physical aspects of humanity. This contempt in turn sometimes led to behavior that was excessively ascetic and theology that ignored the physical or even saw it as evil. Aristotle, however, had disagreed with Plato and had emphasized the importance of the natural, physical world. By adopting a position nearer to Aristotle’s, Aquinas affirmed, for example, that the soul was in close union with the body. This view inevitably elevated the value placed upon the body and the natural world.ix
This greater respect for the physical world had profound philosophical implications. The starting point of the philosophies of both Aristotle and Aquinas was what they perceived with their senses. Thus Aquinas wrote that, “All our intellectual knowledge takes its rise from the senses.”x Both Aristotle and Aquinas believed that all philosophy should start with what we know about existing objects. They believed that there are universal and inescapable common starting points to human thought that are grounded in sense perception, and that these common points are likely to point toward the truth. The philosopher’s task, then, is to identify these common starting points underneath the seemly irreconcilable diversity of human thought and to build conclusions from there.xi This shift in thought led to a greater appreciation of the created world, a perspective which had both theological and cultural significance.xii It helped people to understand Christ’s claim to be divinely human, and it also supported Church sponsorship of scientific research that occurred throughout the Middle Ages and Renaissance.
Aquinas’ work is also steeped in the Aristotelian notion of teleology. Teleology is the idea that everything is directed to some proper goal or end. First Aristotle, then Aquinas, distinguished between the concepts of actuality and potentiality. Aristotle described actuality as the extent to which a being’s potential has been realized and potentiality as the measure of a being’s unrealized potential.xiii Plants have the inherent potential to grow and humans to flourish, and growth and flourishing are the ends toward which their lives are directed. The goal of every life is reached as its potential is actualized. This distinction between potentiality and actuality and its relation to Aristotle’s teleology is prominent throughout Aquinas’ thought, and he used these concepts in many important ways.xiv We take it for granted today that every academic discipline has its own legitimate ends, its own purpose or goal: science to discover truths about the natural world, history to learn about the past, and so on. However, this idea was a matter of debate in Aquinas’ time, as many philosophers thought that the purpose of all disciplines was directly theological. Following Aristotle's teleology, Aquinas maintained that all academic disciplines, while ultimately relating to God, had their own separate and immediate ends.xv This conception of teleology made possible the university as we know it today. Additionally, Aquinas used these ideas to describe God. Aquinas maintained that God was pure actuality without potentiality. Since God is perfect, Aquinas argued that he cannot change or become more, and he is therefore fully actualized. Aquinas believed that God has no teleological end to reach but is instead the end towards which all else is directed.xvi
Another result of Aquinas' interaction with Aristotle was his five proofs for the existence of God, known as the “Five Ways.” These arguments, and modern variations of them, are still discussed and debated today in many introductory philosophy courses. They remain some of the most persuasive proofs for the existence of God ever constructed, and Aquinas shaped his arguments from the philosophical reasoning of Aristotle. One of these five proofs, called the “argument of the unmoved mover,” borrowed the concept of the prime mover from Aristotle. In book twelve of his Metaphysics, Aristotle argued that the fact of the universe demanded the existence of “something which moves others things without itself being moved by anything.”xvii Aristotle believed that the universe was composed of an underlying essence and that the prime mover was the being that moved and organized this essence. Aquinas, centuries later, adapted this argument to argue that all things that move must have been put into motion by something else. However, because motion cannot infinitely regress with no starting point, there must have been at some point something which caused motion in something else but did not itself need a mover. Aquinas called this unmoved mover God.xviii
These examples of Aquinas’ engagement with Aristotle reflect his belief in the harmony of faith and reason and the unity of truth. There are many other examples of Aristotelian concepts and vocabulary in Aquinas’ work, especially in his metaphysics, epistemology, and moral theory. Like the examples above, they are important in their own right. However, more significant still is the fact that Aquinas embraced Aristotle’s work at all. Aquinas undertook the project in spite of objections because he firmly believed that rather than conflicting with each other, the scientific, philosophical, and theological areas of human thought form a whole, integrated, and coherent body of truth. Aquinas' theological system, a synthesis of the truths of the best philosophy and science of the time and the truths of Christian revelation, demonstrates that there is only one truth and that all truth, whether of faith or of reason, is one.
Aquinas once said, “We must love them both, those whose opinions we share and those whose opinions we reject. For both have labored in the search for truth and both have helped us in the finding of it.”xix This statement exemplifies Aquinas’ practice of seeking truth by accepting truth and rejecting error regardless of its source, a practice which empowered him to integrate faith and reason. His synthesis of Christianity with the philosophy of Aristotle continues to influence the trajectory of Christian thought. For example, in recent history theologians utilized Aquinas’ principals to synthesize Christianity and the philosophy of Immanuel Kant. Thomas Aquinas’ work, and the work he continues to inspire even today, demonstrates the intellectual flexibility of Christianity, the oneness of truth, and the convergence of faith and reason.