This post consists of the fruit of some investigations related to a combox discussion on this post. My interlocutor suggests that Schaeffer is correct in proposing that Aquinas paved the way for Descartes. I deny this for the reasons already presented in that post and in the subsequent comments. Here are a few more hopefully helpful bits on the subject. As an aside, I think it’s worth noting that it is Descartes who is regarded as the father of modern philosophy and not Aquinas.
John Peterson writes in Aquinas: A New Introduction, with respect to metaphysics:
[T]he temptation is to conclude that Aquinas was a Cartesian before Descartes. For both philosophers avoid the extremes of materialism on the one hand and idealism on the other. They both deny either that all is matter or that all is mind.
Yet there are important differences between the two philosophers. That is partly due to the fact that Aquinas was less of a Platonist than was Descartes on the matter of persons. For Descartes, a person’s soul or mind is a complete substance, just as it is for Plato. But for Aquinas, who is here closer to Aristotle, a person’s soul is not a complete substance in its own right but rather the form of his or her body. For wider philosophical reasons, Descartes rejected outright the analysis of natural things into form and matter. For that reason, he could not and would not have applied the form-matter schema to the analysis of persons. So even though they are together in denying what is now called identity materialism (as well as, for that matter, epiphenomenalism), the two philosophers part company as regards the sort of thing the spiritual human soul is, i.e. whether it is a complete substance or the (incomplete) form of a substance. [p. xi, available here; emphasis added]
In my view this is a fundamental difference for the present discussion. On the one hand it is consistent both with Descartes’ rationalism (which Aquinas did not share) and with his famous insistence upon starting his philosophical inquiry with himself (or, to be more precise, with his own rational powers)—which Aquinas also did not share.
In a related vein we find the following in The Cambridge Companion to Aquinas. Joseph Owens writes there:
Common to both Aristotle and Aquinas is the tenet that all naturally attainable knowledge originates in external sensible things. By their efficient causality transmitted through the appropriate media, the external things impress their forms upon the human cognitive faculties, and thereby make the percipient be the thing perceived in the actuality of the cognition. The awareness is directly of the thing itself, and only concomitantly and reflexively of the percipient and of the cognitive acts. The external things remain epistemologically prior. From this viewpoint both Aristotle and Aquinas remain radically distinct from modern philosophers, who from Descartes on base their philosophy upon ideas or sensations or vivid phenomena, instead of immediately on external things themselves. Likewise, both Aristotle and Aquinas remain just as distinct from postmodern thinkers who look for their starting points in linguistic and historical formation. [53; emphasis added]
So we see here that Owens too traces the beginnings of modern philosophy not to Aquinas (pace my interlocutor) but to Descartes, and identifies a “radical distinction” between the two.
In the Routledge Encyclopedia of Philosophy article on Descartes, we read the following:
Descartes rejected the Aristotelian philosophy as soon as he left school. … [I]t is probable that what dissatisfied him most in what he had been taught was natural philosophy. [Page 4, which may be seen here]
If Descartes rejected the philosophy, how can it reasonably be said that he was dependent upon Aquinas?
Elsewhere we read:
Descartes did not publish anything until he was forty years old, largely due to his fears of censure.
Why would he fear censure if his views were consistent with those of Aquinas? Answer: he wouldn’t. But he did, because they weren’t.
Descartes self-consciously rejected the philosophical heritage of scholasticism, and attempted to formulate a new philosophical method and construct of new system of philosophical knowledge. It should be noted that Descartes did concede to theology the role that it occupied in the mediaeval period and still occupied in the church of his day; yet justifiably historians attribute this concession to his fear of persecution of ecclesiastical authorities. His statement, “That we must believe all that God has revealed, even though it is above the range of our capacities” (Principles 1.25) is anomalous in his system based on systematic doubt. [Emphasis added]
I think it’s pretty clear that we may safely deny any substantive Thomistic influence on Descartes. Schaeffer was wrong.