In his monumental work The Degrees of Knowledge, Jacques Maritain discusses Descartes’ epistemological mistake (one which he says has been retained by Descartes’ philosophical descendants):
[Modern idealism] is characterized, truth to tell, by a radical misunderstanding of the true nature of the idea and of the intentional function of knowledge, thenceforth conceived upon the pattern of events in the material order. Descartes clearly saw that the known object is known within thought; his capital error was to have separated the object from the thing, believing as he did that the object is in thought, not as an intelligible entity rendered present to the mind through an immaterial form—and with which the mind is intentionally identified—but as an imprint stamped on wax. Henceforth, the intentional function disappears; the known object becomes something of thought, an imprint or portrait born within it; understanding stops at the idea (looked at as an instrumental sign). This idea-portrait, this idea-thing, has as its double a thing to which it bears a resemblance but which is itself not attained by the act of understanding. They are two separate quod’s, and the divine veracity is needed to assure us that behind the idea-quod (which we attain), there is a thing-quod corresponding to it. Of itself thought attains nothing but itself [136-137].Recall with me the Aristotelian-Thomistic understanding of what it means to say that a certain proposition is true: that is, a proposition is true if it corresponds to reality. If I say that the sky is green, everyone knows immediately that what I said is false. It does not correspond to reality. It seems clear from what Maritain wrote that Descartes must necessarily have a different conception of what it means to say that a proposition is true, because for him there is no possibility of a knowledge of the real world around him.
On the one hand, he says this (Maritain’s description of his view is consistent with this):
If I've gotten everything in me from God and He hasn't given me the ability to make errors, it doesn't seem possible for me ever to error. [Source]On that page they quote him saying that “error is a lack,” but of what? It seems that the answer is that truth for Descartes is founded upon “clear and distinct” perceptions (see basically this entire article). Well, clearly this is completely unlike Aquinas’ view (and Aristotle’s for that matter). And if one’s idea of what truth is doesn’t hinge upon correspondence with reality, then what he has done is to functionally set himself free to say that just about anything is true: why not? The effect of this is to remove oneself from accountability for what he believes, because he no longer has any standard by which to measure the truth of what he says. In this there neither is nor can be any dependence upon Aquinas for Descartes, and the two are at odds. St Thomas affirms that what we say must be measured by the standard of reality. Descartes does not. This, it seems to me, is surely at the root of the rise of autonomous reason. But, contrary to Schaeffer, it is absolutely not a view that can be attributed to Aquinas. Schaeffer was wrong. Aquinas cannot be blamed for the disastrous course of modern philosophy.
[Update, a little later] I nearly forgot that I wrote about these subjects a few years ago. In this post, we see that St Thomas more or less addressed Descartes’ erroneous theory about knowledge of the external world, and here is a brief discussion of Aquinas’ views about the reliability of the senses.