Aquinas lived an active, demanding academic and ecclesiastical life that ended while he was still in his forties. 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 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 behaviour 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’ theological writings.
The scope and philosophical character of medieval theology as practised by Aquinas can be easily seen in his two most important works, Summa contra gentiles (Synopsis [of Christian Doctrine] Directed Against Unbelievers) and Summa theologiae(Synopsis of Theology). 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’ 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 works by Aristotle and other authors. Although Aquinas is 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 on any issue.
Aquinas’ 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. There are also in Aquinas’ writings many implicit Aristotelian elements, which he had thoroughly absorbed into his own thought. As a convinced Aristotelian, he often adopts Aristotle’s critical attitude toward theories associated with Plato, especially the account of ordinary substantial forms as separately existing entities. However, 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, more than he was in a position to recognize as such.
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 (Summa contra gentiles I.2.11). Moreover, Aquinas differed from most 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 co-workers in the vast project of philosophical theology, clarifying and supporting 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. However, Aquinas’ philosophy avoids eclecticism 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 skilfully defending, on almost every issue he considers, the most sensible available position.
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