For Descartes, would the following analogy hold?

res cogitans : res extensa :: ens rationis : ens reale

Or did Descartes distinguish the terms res cogitans from ens rationis and res extensa from ens reale?

Scholastic terms:
ens reale = "real being" (a being in the natural world)
ens rationis = "being of reason" (which some consider a non ens or "non-being"); for an in-depth treatment of what entia rationis are, see John Poinsot (John of St. Thomas)'s Tractatus de Signis art. 1 ff.
Descartes' terms:
res cogitans = "thinking thing"
res extensa = "extended thing"


No. For Descartes thinking things (minds) and extended things (bodies) are both what Aquinas and the medieval would have called real beings.

On Descartes's picture minds and bodies are two distinct kinds of substances which are really there, independent of how anybody might think of or imagine them to be. That's what I understand a "real being" in Aquinas's language to be.

What Aquinas means by a "being of reason" is a bit involved, but I can give an example. Suppose I'm thinking of a pie and considering how to slice it--i.e. into quarters, sixths, eighths, or so on. In thinking of the pie this way I'm mentally considering some object, namely possible slices. Those possible slices are "beings of reason" in the sense that their existence is just a matter of my human conceptual activity. They are purely mind-dependent projections onto real beings, which do not posit independent entities really existing outside my mind. And Descartes clearly doesn't think that is true of my mind--my mind exists whether anybody is thinking about it or not, and my mind is not just a mind-dependent projection of some other mind.

  • So, for Descartes, res cogitans and res extensa are both an ens reale? What does Descartes think of ens rationis? – Geremia Aug 19 '16 at 5:16
  • Also, the in-depth treatment of what entia rationis are in John Poinsot (John of St. Thomas)'s Tractatus de Signis art. 1 ff. partially inspired my question. – Geremia Aug 19 '16 at 5:19
  • @Geremia Descartes has a few scattered remarks about what I would guess are supposed to be entia rationis. Look at Meditation 3, AT40-43, where Descartes is arguing from the existence in his mind of an idea of God to the actual existence of God. I think he's saying here that God cannot be a mere entia rationis, because even entia rationis have a kind of real being--as the acts of certain minds, and that real being must have a cause. And since nothing could cause the real being of an idea of an infinite being except an infinite being, therefore God must really exist, extramentally. – shane Aug 19 '16 at 12:31
  • That argument is fallacious, by the way, but I take it that that's what Descartes's argument is. – shane Aug 19 '16 at 12:31

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