What is the relation between Descartes' "lumen naturale", God and logic?

Question

This is not a question about the cogito argument in the closer sense. I will try to isolate my questions from the other questions posted concerning Descartes.

If I understood what I've read correctly, Descartes claims that what he knows clearly and distinctly is true as it can't be any differently. That's true for the cogito, and, let's assume for argument's sake, also for the ergo sum.

Now I read in the Cambridge Companion to Descartes (online available here) that apparently this ergo is no syllogism but an intuition as basic as the cogito itself. My early assumption, namely that Descartes uses logic to infer the ergo sum though he didn't prove logic's truthfulness yet, is thereby rebutted.

But what about inuition? A quote from the Cambridge Companion (p.147):

Second, Descartes' talk of intuition and deduction from intuitions as our two sources of knowledge in the Rules gives way to talk of clear and distinct perception in the Discourse, Meditations, and Principles. He never announces that the faculties are the same, but their equivalence is strongly suggested by the fact that he designates them by similar descriptions: "the light of reason" and "the light of nature." We are told in the Rules that: "intuition is the indubitable conception of a clear and attentive mind which proceeds solely from the light of reason [rationis luce]" (Rule III: AT X 368: CSM 114) and in the Principles that: "the light of nature [lumen naturae] or faculty of knowledge which God gave us can never encompass any object which is not true in so far as it is indeed encompassed by this faculty, that is, in so far as it is clearly and distinctly perceived" (Part I, art.30: AT VIIIA 16: CSM I 203; consider too Meditations: AT VII 38-9:CSM II 26-7).

I already highlighted what I'm aiming at: Even if Descartes doesn't need logic to work before he can actually prove that it does, even if he can "reduce" the ergo sum to a basic intuition - isn't he getting ahead of the argument, relying on the natural light of a non-deceiving god, although it's not before the 3. meditation that he actually proves that there is a god and that he is no deceiver? In other words: If he has not yet proven that it is impossible that the "natural light" we rely on in our knowledge is sent by a deceiving or evil entity, can any step after the cogito be called necessarily true?

To underline the importance of god for knowledge, let me quote one more passage (p.151):

Descartes denies that the atheist has "true knowledge" on the grounds that the atheist is uncertain of whether he is deceived by some god. Prior to proving God's existence and nondeceptive nature, Descartes is just as uncertain as the atheist about the existence of a deceptive god. His clear and distinct perceptions should not produce certainty for him either.

Isn't there at least the possibility that even when we make analytical or deductive judgements, we're mistaken for we're not granted the natural light of the God? Am I missing something? Or is Descartes plainly abandoning his method of "geometrical deduction" and anticipating the proof of God's existence to deduce it?

Answer 1

I don't know what lumen naturale is supposed to be, but here's a guess. Descartes is getting the notion from medieval philosophy, but significantly changing it.

It is an old and important part of the Aristotelian tradition in cognitive psychology that a power cannot err with respect to its object. For instance, the object of the power of vision is color, and, for an Aristotelian, it this power is infallible in the sense that when I am sensing red, then I am sensing red. Error creeps in when I make a judgment like "there is something red". Now the objects of the intellect, as medieval Aristotelians thought about them, were concepts. And again, the intellect is infallible in the sense that when I'm thinking of a square, I'm thinking of a square.

The medieval Aristotelians had a hard time explaining how and why we have knowledge of these abstract concepts like squareness, or horsiness, or whatever. Like Aristotle, they were committed to the view that the only things that exist in reality are particular entities, like this particular square, this particular horse, etc. But if the world is full of particulars, and we get our knowledge from the senses, how does it come to be that we have knowledge of universals like the concept of a square? The short answer is that we arrive there by abstraction--i.e. by examining things that appear to be members of a kind and attempting to isolate the necessary and sufficient features for membership in that kind. On this view, the universals are "in" the individual, particular things insofar as we can abstract away the particularity of the individual member of the kind and get the universal concept.

Now, I said this is what medieval Aristotelians believed. There was also a parallel Platonic tradition that was an important part of the medieval Jewish, Christian and Arabic philosophical traditions. According to the Platonists, the concepts were not "in" the things at all. The sensible particular objects down here are just copies of the perfect ideas of a square, horse, etc. in God's mind, according to which he created everything. We do not learn from sensation, but rather the divine mind illuminates us with its own concepts.

My guess is that Descartes's bit about the lumen naturale is something that he's taken from a 15th or 16th century scholastic philosopher who has tried to syncretize these two traditions. Maybe this person said that our act of abstraction is just what illumination was supposed to be. (Henry of Ghent in the late 13th century had tried to make room for both abstraction and illumination in his philosophical psychology.) If you had some kind of sense that the divine light was just the same as the infallible power of the intellect, maybe that's something you could call a natural light?

Be that as it may, it is clear that Descartes has to be helping himself to much more than this. Because Descartes needs it to be not just simple concepts or sensations which are infallible, but rather whole sentences. The C&D rule is a little plausible if you restrict it to concepts: "when I clearly and distinctly conceive of a square, then I am conceiving of a square" sounds (trivially) true. But there's no reason to think that should work for whole sentences like "when I clearly and distinctly conceive the Goldbach conjecture is false, then the Goldbach conjecture is false." So, even if Descartes is using some medieval language here, he is clearly altering its meaning.