In my beginning study of the history of Western Philosophy (when I was about 16 years old) I had come across a few thinkers that were really quite helpful in getting me started on a philosophical framework for the Christian worldview. One keystone work that was vital in obtaining that framework was Francis Schaeffer’s wonderful book, How Should We Then Live? . An important thing to remember with respect to Schaeffer’s book is (in his words) that “this book is… an analysis of the key moments in history which have formed our present culture, and the thinking of the people who brought those moments to pass” .
Schaeffer is notably associated with the camp of the Presuppositionalists, which, for the sake of reference, we can define as someone who “assumes the biblical revelation in all matters relating to conceptions of reality and logic…” In order “to demonstrate to the unregenerate, who is using irrational methods and distorted truth… that only biblical presuppositions provide the necessary means to make sense of the world and life” . This is a central idea in Schaeffer’s book.
Francis Schaeffer and Basic Worldviews
In the opening chapter of Ancient Rome in his book, Schaeffer points out that “people have presuppositions” . These presuppositions we can define as “the basic way an individual look at life, his basic world view, the grid through which he sees the world.” As this is a point throughout his book which exemplifies itself in sometimes noticeably explicitly ways, Schaeffer comes across in his discussion of the Middle Ages the ‘supposed’ view of Thomas Aquinas that “man had revolted against God and thus was fallen, but… the Fall did not affect man as a whole but only in part” (p. 52). After a walk through discussion regarding Aristotle and the universal/particular distinction, Schaeffer writes:
Thomas Aquinas brought the Aristotelian emphasis on individual things – the particulars – into the philosophy of the Middle Ages, and this set the stage for the humanistic elements of the Renaissance and the basic problem they created. (p. 52)
The clear theme of this post is to say that Schaeffer was just completely mistaken on this point on a rather fundamental level. His evaluation of the supposed mistakes of Aquinas’ “Aristotelianizing of Christianity” (or “Christianizing of Aristotle”, should you prefer) is a misrepresentation of Aquinas’ inclusion of Aristotle’s philosophy as a “classical synthesis” between theology and philosophy. The following section will exposit Aquinas’ true understanding of the matter.
Aquinas’ Nature-Grace Distinction
As R.C. Sproul writes in his volume of Western Philosophy, The Consequences of Ideas (2000), “Perhaps no Roman Catholic thinker has been more maligned, misunderstood, and misrepresented by Protestant critics, especially evangelical critics, than has Thomas” . In light of Schaeffer’s charges, Sproul continues:
It is widely accepted that Thomas’s most egregious error was to separate nature and grace. This charge is unmitigated nonsense; nothing could be further from the truth. To charge Thomas with separating nature from grace is to miss the primary thrust of his entire philosophy, particularly with respect to defense of the Christian faith. (Ibid.)
First, an explanation of this matter is in order. Schaeffer explains the issue as Grace, the higher: “God the Creator; heaven and heavenly things; the unseen and its influence on the earth” . The other distinction being, Nature, the lower: “the created; earth and earthly things; the visible and what happens normally in the cause-and-effect universe” (Ibid.). Schaeffer then has this to comment:
Thanks to Thomas Aquinas, the world and man’s place in the world was given more prominence than previously. The negative result of his teaching was that the individual things, the particulars, tended to be made independent, autonomous, and consequently the meaning of the particulars began to be lost. (Ibid. p. 55).
In Aquinas’ great and volumous work, the Summa Theologiae (“Sum Total of Theology”), biographer Diarmaid MacCulloch writes that the “Summa deals with the most abstract questions of being and the nature of God, yet it also extends to very practical discussions of the way everyday life should be viewed, and how we should live as a part of God’s purpose. Through its questions and distinctions pushing to conclusions, it presents a harmonious view of God’s earthly and heavenly creation… Thomas put limits on the use of reason in understanding this harmony” .
This is in almost direct contrast to Schaeffer’s analysis of Aquinas! Earlier on in MacCulloch’s exposition of Aquinas he explains how he “took as the grounds of his work that the systems of thought and reasonable analysis presented by Aristotle did not deny the central places of faith, but illustrated, even proved, its truths” . Aquinas’ view of nature and grace was not that they were separate areas of truth, or under a Muslim construction of “double truth”; but rather that there was a distinction between the faculties of nature and grace. This Muslim construction of “double truth” that I refer to has to do with particular Muslim philosophers – such as Averroes and Avicenna – that say certain domains of truths may be the case in one but not in the other. For example, what may be true in philosophy might also be false in theology, and even vice versa. Sproul comments that this “intellectual schizophrenia separates nature and grace with a vengeance” .
Thomas’s contrary view to this was that he believed philosophy and theology “play complementary roles in the quest for truth. Grace does not destroy nature but fulfills it” (Ibid. p. 69 – emphasis mine). To scheme Aquinas’s views in a rather full manner, consider the following:
- Aquinas adopts a philosophical approach to faith.
- Aquinas tends to regard faith as relating to propositions about God.
- Aquinas relates faith to evidence.
- Aquinas’ notion of faith is theological, in that it relates to God himself. 
To finish on a point from Alister McGrath:
Aquinas adopts a strongly intellectual approach to faith, treating it as something which is mid-way between knowledge (scientia) and opinion. For Aquinas, scientia has the sense of “something which is self-evidently true”, or “something which can be demonstrated to be derived from something which is self-evidently true.”
In the case of scientia, true compels assent on the part of the human intellect either because it is self-evidently correct, or because it is supported by such powerfully persuasive logical arguments that no rational mind could fail to be convinced. In the case of faith, however, the evidence is not sufficient to compel the human intellect to accept it… The commonsense understanding of faith, as a lower form of knowledge, thus seems to be well grounded in the writings of Thomas Aquinas. 
-  Francis Schaeffer, How Should We Then Live? (Crossway Books: 1976)
-  Ibid., p. 15
-  H. Wayne House, “Biblical Argument for Balanced Apologetics,” in Reasons for Faith, ed. Norman Geisler and Chad V. Meister (Crossway: 2007) p. 57
-  Francis Schaeffer (1976), p. 19
-  R.C. Sproul, Consequences of Ideas (Crossway: 200) p. 67
-  Schaeffer (1976), p. 55
-  Diarmaid MacCulloch, Christianity: The First Three Thousand Years (2009) p. 413 – emphasis mine.
-  Ibid., pp. 412-413 – emphasis mine
-  R.C. Sproul (2000), p. 68
-  Taken from Alister McGrath, Introduction to Christian Theology (Blackwell Publishers: 2001) p. 241
-  Ibid., pp. 238-239