Descartes seems to be a bona fide dualist, that is, he asserts that at there are two different kinds of things: those that think and those that are extended. Such a dualism might well have been unintentional- for dualism, if it means anything, means speaking about two fundamentally different sorts of thing, and it is not clear that Descartes used the word “thing” with any sophisticated or reflective account of it. “Res” (thing) is simply demanded by the mode of signification, since extensa and cogitans are adjectives that sound clunky when used substantively. Descartes only seems interested in the assertion that there are thinkers and “extendeds”, and that what is one is never the other.
I’m not aware of any moment when Descartes notices that there can be one thing that is not another without there being two things. That sentence is not a contradiction, just as the title of this post is not, for the first “thing” is used in an extended and derivative sense while the second one is used in a primary sense. The parts are not the whole, nor are substances accidents, but this division does not give rise to two things. If someone asks you what one thing you would want to have on a desert island, and you say your wife, he can’t object by saying that this would mean taking two things: your wife and her left pinkie toe. Similar considerations apply to all parts and wholes. The logical whole and part (like “rational” and “animal” in man) doesn’t make the thing defined two things, nor does the ontological composition of form and matter. Thing means something before philosophers get to it, and it is simply not the case that we can call the term of any division two “things” unless we qualify in one way or another that we are using “thing” in an extended sense, and not in the primary sense.
This division matters because we are prone to reify, or literally “thing-ify”, the terms of any division, but this is a trick of false imagination. Venn diagrams make “animal” a kind of whole, whereas the term as the logician uses it has more the character of a part- that is, a part of a definition. (In fact the logical whole is in very important ways contrary to the integral or extended whole: for the extended whole is complete whereas the logical whole is confused and incomplete.) Even if we use the word “thing” for the product of all these divisions, this is no guarantee that we are speaking of what we mean by “thing” in an unqualified sense. In the absence of a qualification, we can’t use “thing” as though it were a sticker that can be slapped on the terms of any division whatsoever.
St. Thomas’s way of articulating this is that a substance, or “thing” in an unqualified sense, requires not just being set apart from some other (even where this other can exist separately), but also a complete or whole existence. This is not a mere semantic quibble, but a response to a legitimate problem that not every division- or “this is not that”- gives rise to a new whole. Thing requires both being set apart and being somehow whole. We can, of course, also give things a sort of whole existence in our mind that they don’t have in things (to use the word in two other ways) but to do so is to more make a statement about how we think than about the way things are.
We tend to use the word “thing” as if it were a rather unsophisticated concept, but a closer look at it shows a good deal of structure that is easy to miss. Our minds have spontaneously formed a much more complicated and sophisticated account of “thing” than we are prone to recognize. This is to be expected.