Aquinas writes in Summa Contra Gentiles 2.4.1 that “the Christian faith deals with creatures so far as they reflect a certain likeness of God,” whereas “human philosophy considers them as they are…. The Christian faith, however, does not consider them as such; thus, it regards fire not as fire, but as representing the sublimity of God.” As Thomas continues in the following paragraph (2.4.2),
For this reason, also, the philosopher and the believer consider different matters about creatures. The philosopher considers such things as belong to them by nature–the upward tendency of fire, for example; the believer, only such things as belong to them according as they are related to God–the fact, for instance, that they are created by God, are subject to Him, and so on.
In hermeneutical terms, philosophy interprets things literally, theology allegorically. (In more historical categories, philosophy is Aristotelian, theology Platonic.)
Some thoughts in response. First, even on Aquinas’s own terms this division of labor seems to involve a bit of an equivocation on the meaning of philosophy. As Thomas explains elsewhere, philosophy itself is nothing other than the knowledge of God, nor is he always careful to limit philosophy’s knowledge of God to that which reason alone can know apart from faith (which is exactly how he characterizes philosophy in the opening question of the Summa Theologiae). As Thomas explains in the prologue to the Summa Contra Gentiles, for example, ultimate wisdom or sophia, which philosophy is the love (philos) of (and which Aquinas identifies as the subject matter of the present work) includes both those truths which are within the grasp of reason and those truths which surpass reason’s reach. The pursuit of wisdom, in other words, must inevitably come to include both the rational and the supra-rational, and hence both a natural and a revealed theology. For Aquinas, by reason man knows that God exists, by reason he knows that the knowledge of God is man’s end, and yet by reason man knows reason’s own incapacity for knowing God as he is in himself, which means that by reason man knows his own need for revelation if he is to reach his end (and we haven’t even factored in the pervasiveness and perversity of sin yet). All of this is to say that there is a very real sense for St. Thomas in which philosophy, rightly understood and properly exercised (which he virtually admits it almost never is) demonstrates to itself man’s need for, and hence philosophy’s own openness to, revelation. On either of these two understandings of philosophy–i.e., philosophy in the etymological sense of the “love of wisdom” and which encompasses both natural and revealed theology, and philosophy in the narrower sense of a natural or rational theology as distinguished from revealed theology–then, Thomas’s above, immanentized and secularized definition of philosophy as a knowledge, not of God, but of things as they are in themselves and in their own nature, seems strangely limited.
By philosophy, therefore, Thomas in the setting of SCG 2 would seem to mean something like natural philosophy, including physics and biology. And while this would seem to be in keeping with the subject matter of SCG 2 as a whole, which is creation and created things, it does raise the question as to what kind of treatment Thomas is proposing to give of creatures in SCG 2: is it a “theological” (i.e., God-centered treatment) or is it going to be a “philosophical” (i.e., scientific, thing-centered) treatment, one that effectively brackets the creature’s otherwise inherent orientation towards God? The answer, given the subject matter and purpose of SCG 2, is clearly the former: Thomas is here interested in creatures only so far as they relate to God. Yet the irony is that the treatment Thomas will give of creatures in SCG 2 is, at the same time, purely philosophical in the sense that his methodology is an exclusively rational one (i.e., he will demonstrate all of his conclusions about creation philosophically), and at no point is his argument formally dependent upon revelation for any of his premises or conclusions (though Thomas will certainly invoke Scripture and other authorities to corroborate his conclusions). Yet if what Thomas in fact offers in SCG 2 is a (rational) theological account of creation, instead of a (natural/scientific) philosophical account of creation, why does he he introduce his discussion by talking about how the “Christian faith deals with creatures so far as they reflect a certain likeness of God”? And why does he go on to contrast how “the philosopher and the believer consider different matters about creatures”? On Thomas’s own terms, the juxtaposition here ought not be between the believer and the philosopher, but between the philosophical theologian and the natural scientist.
And this leads me to my final thought, which is that even if we take Thomas’s juxtaposition here to be between the believer and the philosopher (which I have just shown not to make any sense in light of his own project), I’m not sure that we should agree with his conclusion. I am comfortable with a division within rational philosophy between a philosophical theology on the one hand and a philosophy of nature on the other: there is a difference, for example, between the way in which the Christian scientist, qua scientist, looks at creation in his laboratory and the way in which the Christian philosophical theologian, qua philosophical theologian, looks at creation from his armchair. So long as they are both doing what they are doing to the glory of God, I don’t that we need to insist that they are therefore doing the same thing to the glory of God. And if this is all that Thomas means (or means to mean), then I don’t see a problem. However, if Thomas means, in his references, for example, to the “Christian faith” and to the “believer,” to suggest that the Christian qua Christian looks at creatures differently than does the natural scientist qua natural scientist, then I think we need to beg to differ. (For one thing, “Christian” has no real counterpart except “non-Christian,” or “believer” except “non-believer,” so the contrast between “believer” and “philosopher” by itself doesn’t make any sense.) In question 1 of the Summa Theologiae, Aquinas argues that sacred doctrine (revealed theology) was made necessary “for man’s salvation” (though as we have seen, Thomas indicates a sense in which it would have also been necessary apart from the fall, namely in wisdom’s pursuit of those truths about God which surpass the grasp of his reason), but there is an alarming tendency in Aquinas, I find, to view this need for salvation as something that primarily pertains to man’s future life (in this sense Aquinas is already on his way to Descartes, who in his Discourse on Method limits theology’s usefulness to that of “teach[ing] one how to reach heaven”). But if man needs salvation, then this is a need that touches on every aspect of his life: if man as man (and not just “eschatological man”) needs redeeming, then so does man as philosopher, as scientist, as car-mechanic, as anything. This means that man as Christian or believer, which is to say, man as redeemed, will have the same interest in created things as any other office he might hold, including that of the philosopher or scientist. It is for this reason, and as Cornelius van Til memorably put it, that the Bible is authoritative in all that it speaks to, and it speaks to everything. And as Augustine observes in De doctrina Christiana, because Scripture speaks of the natural world, some knowledge of the natural world will be necessary to adequately understand Scripture. This is to suggest that even man as bible-reader will not necessarily have a lesser interest in or curiosity about things as they exist in themselves or in their own nature than that possessed by man as scientist. Indeed, it may very well guarantee that his interest and curiosity is all the greater.