The interrelation and integration of faith and reason was at the heart of medieval intellectual life in general and of the thought of St. Thomas Aquinas in particular, and indeed, it was St. Thomas’s statement on the relationship between faith and reason that Pope Leo XIII made virtually the official position of the Roman Catholic Church in his 1879 encyclical Aeterni Patris (which you can read here). One of the things Pope Leo particularly commended in St. Thomas was his “clearly distinguishing, as is fitting, reason from faith, while happily associating the one with the other,” and thus “both preserved the rights and had regard for the dignity of each…” Some of Thomas’s more familiar statements along these lines appear in the opening articles of the Summa, in which he emphasizes the distinct spheres in which faith and reason have their proper operation. In the very first article of the Summa (“Whether, besides philosophy, any further doctrine is required”), for example, Thomas explains how Scripture, which is believed on by faith and which forms the basis of sacred doctrine or theology, “is no part of the philosophical sciences, which have been built up by human reason” (ST 1.1.1 sed contra). The need for sacred doctrine lies both in man’s need for salvation and in the fact that man is by nature directed towards God as to an end that surpasses the grasp of his reason. In the second article of the second question (“Whether it can be demonstrated that God exists”), Thomas further writes: “[t]he existence of God and other like truths about God which can be known by natural reason, are not articles of faith, but are preambles to the articles. For faith presupposes natural knowledge, even as grace presupposes nature…” (ST 1.2.2 ad 1). Not only are faith and reason distinct modes of knowing, and therefore theology and philosophy distinct sciences or bodies of knowledge, there is a sense in which reason and philosophy even exercise a certain epistemic priority or at least immediacy over faith and theology, insofar as the latter “presuppose,” build upon, and to that extent may be said to depend upon the former. For Pope Leo and the tradition of Leonine Thomism stemming from him, accordingly, much of the virtue of St. Thomas’s position lay in his giving both faith and reason their due and his acute sense for distinguishing and not confusing the distinct domains or spheres in which these two have their proper operation.
What resulted from Pope Leo’s encyclical, however, was not so much a revival of Thomism as a revival of Thomisms, a fact that has suggested to some that Thomas’s distinction between faith and reason may not be quite as clear-cut or straightforward as has sometimes been assumed, and Aquinas scholar Rudi te Velde has gone so far as to state that “[t]he subtle balance and interplay between reason and faith, philosophy and theology, in the Summa undoubtedly constitutes one of the main challenges for any serious interpretation of Aquinas’s thought.” As Velde summarizes, whereas traditional approaches to Thomas attributed to him a natural theology still very much conceived along the rationalist, foundationalist lines of secular philosophy, more recent approaches to Aquinas have gone to the opposite extreme in depicting him “with distinctively Wittgensteinian traits in repudiating philosophical proof and rational foundations,” so that even his famous five ways have been reconceived as not so much objective proofs for God’s existence as insular (and somewhat insipid) “ways of thinking about God from the perspective of the Christian belief in God.” What both of these positions hold in common, despite their differences, Velde contends, is a shared, unquestioned faith-reason dualism according to which philosophical reason “aim[s] at a universal objectivity of rational justified truth, which requires a suspension of the ‘subjective’ claims of a particular historical revelation.”
Other interpreters of Aquinas have sought to re-read Aquinas in a way that overcomes or avoids the imputation of faith-reason and related dualisms to Aquinas’s texts, and to challenge the idea of an autonomous or independent “secular” reason which might be pitted over against faith and divine revelation. Velde in particular has countered that the divine revelation of faith for Aquinas consists in something far more than a mere positivist, factual claim that “escapes any verification by reason,” but constitutes instead an entire body of truth invested with its own intrinsic, intelligible meaning, a point illustrated, for example, in Thomas’s insistence that, while sacred doctrine does not depend upon the philosophical sciences to prove any of its principles, it can and does “take from the philosophical sciences… in order to make its teaching clearer” (ST 1.1.5 ad 2). Theology and faith are thus capable of and receive a “manifestatio through philosophy,” that is to say, “a rational clarification of a truth of revelation by means of philosophical arguments.” Velde summarizes Thomas’s purpose in the opening passages of the Summa in these words:
The truth of Christian doctrine is not simply taken for granted, but neither does he attempt to prove its divine origin and, consequently, its truth from the external standpoint of reason… [F]or Aquinas, there is no such external standpoint from which the way reality is pictured in the Christian tradition might be compared to reality itself. Aquinas places himself within the particular tradition of Christian faith, not simply by identifying himself with the particularity of its “truth,” but by arguing for the intelligibility of the Christian self-understanding. In this way he opens a universal perspective of truth, from within the particular tradition of Christianity, in so far as he aims to show that the notion of revelation has an intelligible sense.
According to Velde, then, we do wrong to think of Thomas as assuming an autonomous, neutral rationality from which a “pure” natural theology might be constructed, even if it should leave room for, and even need completing by, a later revealed theology supernaturally added by grace. Rather, for Thomas, Christianity represents a coherent, integrated worldview comprised of both a natural and a revealed theology, of both a way of faith and a way of reason, which are to be taken together and whose coherence or intelligibility must be appreciated not from some supposed outside, “objective” perspective, but “from the inside,” as it were. In this way, as we shall see more fully later, the natural theology of Thomas’s Summa, while not written specifically for the unbeliever, nevertheless has application for him. It makes a rational claim upon the non-believer, in other words, without on that account making a concession to the purported autonomy of his rationality.
To consider further some of the ways in which Thomas challenges the faith-reason dualism endemic in much modern thought, I want to examine in greater detail one of Thomas’s much earlier discussions of faith and reason from his commentary on the sixth-century Boethius’s De Trinitate. In this work, dating from the beginning of his first regency at the University of Paris, Thomas accords a more prominent place to St. Augustine’s famous theory of “divine illumination” in the act of knowing than he characteristically does in later works such as the Summa. According to St. Augustine, all human knowledge, including that of sense-particulars, is made possible by the mind’s being directly and supernaturally illumined from above by God. As Augustine writes in his Literal Meaning of Genesis, just as “air has not been given its own luminosity, but it becomes luminous” through the action of light, “[i]n a similar way, man is illuminated when God is present to him, but when God is absent, darkness is immediately upon him…” In the Summa, as is well known, Aquinas associates Augustine’s doctrine with its Platonic antecedent, contrasting, on the one hand, Augustine’s and Plato’s position that “intellectual knowledge is not brought about by sensible things affecting the intellect, but by separate intelligible forms being participated by the intellect,” with Aristotle’s theory of abstraction (favored by Thomas), on the other hand, according to which it is the agent intellect, immanent within and natural to the individual perceiver, that sheds the intelligible light by which sensible things are known (ST 1.84.6). In his commentary on Boethius’s De Trinitate, however, Thomas appears more openly Augustinian, presenting as he does the natural light of the agent intellect, not in contrast with Augustine’s divine illumination, but more in terms of a naturalized form of that illumination. Thus, in an article defending the integrity of the natural powers of human reason to know truth, Thomas states that “the human mind is divinely illumined by a natural light” (EBT 1.1). Thomas further emphasizes that the divine gift of the natural light of the intellect is not a one-time, once-for-all donation, as though after positing the intellect in its existence God then leaves it to itself. Instead, as Thomas comments on the above passage from Augustine’s Literal Meaning of Genesis,
just as air is illuminated by the presence of light, which in its absence leaves air in continual darkness, so also the mind is illuminated by God. God is always the cause of the soul’s natural light—not different lights but one and the same. He is the cause not only of its coming into existence but of its existence itself. In this way, therefore, God is constantly at work in the mind, endowing it with its natural light and giving it direction. So the mind, as it goes about its work, does not lack the activity of the first cause. (EBT 1.1 ad 6)
Here we have the epistemological application of Thomas’s more general metaphysical doctrine of divine concurrence: the natural powers of the human mind are not set over against or separated from God’s own power and operation, but are rather viewed as a particular, regular, integral, dependable, and natural form of that power and operation. Natural knowledge, in short, is already in a sense “supernatural,” thus guaranteeing its mutual compatibility and support with the divine light of faith.
More than their mere compatibility, however, and the Summa’s later claim that “faith presupposes natural knowledge” notwithstanding, Thomas further suggests in his commentary on Boethius that there is a sense in which reason in fact must presuppose faith. Having shown that a “new,” supernatural illumination is not necessary for the human mind to know truth by its own power, Thomas comes back in a later article to ask in what sense faith nevertheless might be “necessary for the human race” (EBT 3.1). One of his arguments here points to a profound irony and tension in the order of human knowledge: although we only come to know what is most knowable in itself (namely the uncreated “divine and necessary realities”) through a consideration of what is known “first for us” (i.e., those created, sensible objects of nature), it is only in their relation to what is known last (i.e., God) that what is known first (i.e., creatures) is given its ultimate ground of intelligibility or truth. Thus, unless some knowledge of these divine and necessary realities is given and intuited from the outset, no such intellectual ascent could ever properly get underway. Thomas states the problem and its solution in these words:
But what we first know is known on the strength of what we eventually come to know; so from the very beginning we must have some knowledge of those things which are more knowable in themselves, and this is possible only by faith. The sequence of the sciences makes this clear, for the science that concerns the highest causes, namely metaphysics, comes last in human knowledge, and yet the sciences that precede it must presuppose certain truths that are more fully elucidated in that science. As a result, every science has presuppositions which the learner must believe. Consequently, since the goal of human life is perfect happiness, which consists in the full knowledge of divine realities, the direction of human life from the very beginning requires faith in the divine, the complete knowledge of which we look forward to in our final state of perfection. (EBT 3.1, emphasis added)
If faith presupposes reason in the opening of the Summa, there is another sense, at least for the Thomas of the commentary on Boethius, in which reason at the same time, and perhaps even more deeply, presupposes faith.
In summary, then, the picture of the relationship between faith and reason that emerges is not that of two divergent modes of knowing which are isolatable into distinct, compartmentalized, and mutually-exclusive noetic intervals. Rather, what they represent are two coordinate rays of “light,” simultaneously radiating from a shared, divine source, each of which serves in its own way to illuminate the human intellect, and in doing so serving to illuminate and support the other. Faith thus not only completes reason by offering the promise and guarantee of that which reason desires but cannot on its own attain, namely the vision of God in his essence, but is also what sets reason on the right path in the first place by providing it with its initial impetus and orienting trajectory towards the divine. At the same time, it is this faith-informed reason which first enlightens man as to his natural limits and consequent need for something beyond reason, and afterward helps explicate the inner intelligibility of faith once faith has arrived. Faith and reason thus continually key-off of and give traction to each other as the intellect makes its incremental ascent along the path towards its final end of the beatific vision of God, when both faith and reason are at last superseded by the unmediated, intuitive experience of God in his essence. For this reason John Milbank has suggested that, for Aquinas, “beneath the distinction of fides and ratio along our temporal ways, lies the much more fundamental contrast between in patria and in via” in which “both faith and reason are dim anticipations of the final vision of glory.”
 Garcia, “Philosophy in the Middle Ages,” 3-5.
 Leo XIII, Aeterni Patris.
 “Scriptura autem divinitus inspirata non pertinent ad philosophicas disciplinas, quae sunt secundum rationem humanam inventae.”
 “Deum esse, et alia huiusmodi quae per rationem naturalem nota possunt esse de Deo… non sunt articuli fidei, sed praeambula ad articulos: sic enim fides praesupponit cognitionem naturalem, sicut gratia naturam…”
 Velde, “Understanding the Scientia of Faith,” 53.
 Ibid., 56-7. For a brief history of the interpretation of Thomas’s five ways, see Kerr, “Ways of Reading the Five Ways” in After Aquinas.
 Velde, “Understanding the Scientia of Faith,” 58.
 See, for example, Milbank, “Truth and Vision.”
 Velde, “Understanding the Scientia of Faith,” 59.
 “[H]aec scientia accipere potest aliquid a philosophicis disciplinis, non quod ex necessitate eis indigeat, sed ad maiorem manifestationem eorum quae in hac scientia traduntur.”
 Velde, “Understanding the Scientia of Faith,” 71.
 Ibid., 60. As Velde explains this interpretation as it applies to Thomas’s five proofs for God’s existence, “[w]hat [Thomas] is saying is this: although there are several objections against the existence of God, which should be taken seriously, we Christians affirm, by the authority of Scripture itself, that God exists. Assuming that this is true, as we believe it is, let us try to show, through reason, how this truth that God exists can be made understandable to us.” Ibid., 71.
 As Victor White observes, the Summa is “for Catholics. It is not immediately intended for the unbeliever or the atheist. Truly…, it must for that very reason be concerned to show how unbelievers are to be taught, that is to say…, led from what they do know to what they do not… It in no way substitutes a ‘natural theology’ for revelation, nor does it appeal to reason for what only revelation can impart. But it is part of its own task to teach those who acknowledge no revelation at all…” White, “Prelude to the Five Ways,” 26, 32.
 Torrell, Saint Thomas Aquinas, Volume 1: The Person and His Work, 67. All quotations from Thomas’s Expositio super librum Boethii De trinitate (EBT for short) are taken from Thomas Aquinas, Faith, Reason, and Theology: Questions I-IV of his Commentary on the “De Trinitate” of Boethius, trans. Maurer.
 Augustine, The Literal Meaning of Genesis 8.12.26.
 As Gareth B. Matthews observes, “the chief ancient rival to the doctrine of illumination is the Aristotelian idea of abstraction.” Matthews, “Knowledge and Illumination,” 181.
 As John Milbank has aptly described the process, the “Augustinian and Neoplatonic construal of truth as inner illuminatio” hasn’t been so much replaced by Aquinas as rerouted through an “Aristotelian detour through the truth embodied in finite creatures and conveyed to us only via the senses.” Milbank, “Truth and Vision,” 23.
 “[S]icut dicit Augustinus VIII super Genesim, sicut aer illuminatur a lumine praesente, quod si fuerit absens continuo tenebratur, ita et mens illuminatur a Deo. Et ideo etiam lumen naturale in anima semper Deus causat, non aliud et aliud, sed idem; non enim est causa fieri eius solum, sed etiam esse illius. In hoc ergo continue Deus operatur in mente, quod in ipsa lumen naturale causat et ipsum dirigit, et sic mens non sine operatione causae primae in operationem suam procedit.”
 “Sed quia ex vi illorum, quae ultimo cognoscimus, sunt nota illa quae primo cognoscimus, oportet etiam a principio aliquam nos habere notitiam de illis quae sunt per se magis nota; quod fieri non potest nisi credendo. Et etiam hoc patet in ordine scientiarum, quia scientia quae est de causis altissimis, scilicet metaphysica, ultimo occurrit homini ad cognoscendum, et tamen in scientiis praeambulis oportet quod supponantur quaedam quae in illa plenius innotescunt; unde quaelibet scientia habet suppositiones, quibus oportet addiscentem credere. Cum ergo finis humanae vitae sit beatitudo, quae consistit in plena cognitione divinorum, necessarium est ad humanam vitam in beatitudinem dirigendam statim a principio habere fidem divinorum, quae plene cognoscenda exspectantur in ultima perfectione humana.”
 As Velde writes, “without a sort of revelation by which the ultimate goal of human life is disclosed in the manner of a promise, thus as beyond what is humanly attainable, the infinite aspiration, which underlies the process of humanization, would always be frustrated. Revelation is necessary in so far as it offers human life in this world a concrete orientation to the transcendent horizon of the good and the true.” Velde, “Understanding the Scientia of Faith,” 62.
 Milbank, “Truth and Vision,” 36.