According to Descartes' demon hypothesis, would it be possible for the demon to deceive us about the rules of logical inference e.g. could my belief in the law of non-contradiction be caused by the demon?

I think that once Descartes goes beyond the dream to the demon, we could be wrong about anything. It's not that the demon could change the laws of logic: according to Descartes, I believe, not even God could do that. But the demon could make the simplest logical truths seem false to us and the most blatant logical falsehoods seem true. This is what he calls 'hyperbolic doubt', a beautiful expression for a nasty situation. What is mysterious is how he thinks that even 'cogito ergo sum' can survive this kind of warp-drive skepticism.

Gosh, I disagree. Descartes seems to argue that it is God who makes the eternal truths eternal, by fashioining our minds so that we cannot doubt them.

I'm no Descartes scholar and Jay may well be right that actually Descartes held that God makes the laws of logic true, or neccessarily true. But the answer to the question still seems to be that, for all Descartes knows in the First Meditation (before he has convinced himself of the existence of God), he could be wrong even about those laws, and that would be so even if the laws of logic were beyond the control of God or demon.

Again, this awaits a Descartes scholar, but what Descartes says in his "First Meditation" is this:

Andbesides, as I sometimes imagine that others deceive themselves in thethings which they think they know best, how do I know that I am notdeceived every time that I add two and three, or count the sides of asquare, or judge of things yet simpler, if anything simpler can beimagined?

So it seems Descartes argues that, as far as we can tell by the end of the "First Meditation",we could be wrong about (and so can't be said to know) basiccomputational facts, wrong about (what philosophers often call) analytictruths (like "A square has four sides"), and even wrong about "yet simpler"matters (like logical laws, perhaps).

Jay is correct that eternal truths are up to God. In a letter to Mersenne, Descartes says that "since God is a cause whose power surpasses the bounds of human understanding,and since the necessity of these truths does not exceed our knowledge, these truths are therefore something less than, and subject to, the incomprehensible power of God."

Nevertheless, Peter is quite right--in virtue of the textual evidence that Alex cites--to say that the evil deceiver (or 'omnipotent God') doubt is introduced in order to cast mathematical truths into doubt.

It's worth noting, however, that in the Third Meditation, this doubt about eternal truths is characterized as "slight and metaphysical". Indeed, Descartes writes: "Yet when I turn to the things themselves which I think I perceive very clearly, I am so convinced by them that I spontaneously declare: let whoever can do so deceive me, he will never bring it about that I am nothing, so long as I continue to think that I am something; or make it ture at some future time that I have never existed, since it is now true that I exist; or bring it about that two and three added together are more or less than five, or anything of this kind in which I see a manifest contradiction" (italics added).

Descartes' point, then, seems to be the following: one cannot doubt one's occurrent perception of eternal truths, but, as long as one does not know that God exists and is not a deceiver, one can have doubts about one's non-occurrent perceptions of eternal truths. Consequently, in order to be able to rely on one's (clear and distinct) perceptions of eternal truths when one is not actually, occurrently, having such a perception, one needs to prove that God exists.

This is important, because when one is doing math or science, one cannot hold all the steps of a proof in one's mind, and so one must have the assurance that one cannot be deceived in thinking that one's earlier perception of an eternal truth was true.

May I weigh in a bit? I think that panelists are right to suggest that while the dream argument addresses the veracity of perception about the world, the demon argument goes farther and addresses mathematical and logical inferences. I'd like, however, to return to Peter Lipton's question about why the cogito survives the demon argument. There's a bit more to say on that score that explains Descartes's position.

There's a difference between mathematical and logical inferences and the cogito, and that is that what Descartes finds persuasive about the cogito is not a matter of inference but rather direct intuition. The demon argument works but pointing out that in any discursive movement of thought, from one idea to another, the demon might interfere. Discursive thought, therefore, is dubious. N.B. that's why the example Alexander George quotes is about discursive movements like "adding" and "counting." In this way, Descartes anticipates, in a way, the point Quine makes in his article, "Two Dogmas of Empiricism" with regard to analytic relations--viz., that the connection between one word and another cannot be coherently understood as an internal relation like "containment." (By "internal relation" I mean that the relation is somehow essential or contained by the ideas, that the ideas could not exist as they do without it.)

But the cogito is a different matter. While sometimes Descartes presents the cogito as a sort of discursive inference (as in the popularizing Discourse with "I think, therefore, I am"; je pense donc je sui; he never says "cogito ergo sum"), note that in the Meditations his formulation is focused: "I think. I exist." There's no "therefore", no discursive movement of thought, just direct inspection, intution via what he calls the "natural light" (lumens naturales).

To see the real importance of this: In his epistolary works (somewhere, I forget just now) indicate, Descartes distinguishes the verum (true) from the certum (certain). This is an important distinction. Here's why: What captures him first about the cogito is that it is certain; it can't be doubted. (Remember the title of the First Med is not concerned with what is erroneous but what can be doubted.)

The reason it can't be doubted is not that it's correspondence to the world, or some other external relation, is certain but that it has various INTERNAL properties that render it indubitable--namely that it is "clear" and "distinct." (Similarly, Spinoza articulates his conception of "adequacy" as a property of the idea itself rather than its relation to something outside ideas.) (A contingent relation, btw, is one that is brought to it from outside itself and can be changed without changing the idea.)

That's what ultimately makes the so-called rationalists what they are: the attempt to determine things on the nature of what's in the ideas themselves, what' internal to them, and their internal relations to one another. (That's why Hume's philosophy is so devastating to the rationalists--for Hume all relations among ideas are external and there's nothing in ideas themselves to render them indubitable.)

Descartes view on clear and distinct ideas is a reworking of the ancient stoic's doctrine of "cataleptic impressions." Ancient academic skeptics and stoics argued about whether or not there are any ideas that are self-verifying and beyond doubt. Stoics held that there are; skeptics held that there aren't. Similarly, Descartes argues that clear and distinct ideas cannot be doubted; Hume holds that no ideas are immune to skeptical doubt (with a curious twist, I'll grant you, in the case of geometry and math).

Where, then, does truth come in for Descartes? Well, notice that he doesn't prove the "real distinction" between the mind and the body as well as the claim that any of our ideas about the world are true until Meditation 6. Certainty comes with the "clear and distinct" criteria disclosed in the cogito. But only two ideas contain both their certainty and truth: the cogito (I think. I exist.) and God. The very idea of God, like the cogito, is both certain and self-verifying. But internal to the idea of God (or contained in the idea of God) is something else not contained in the cogito--the foundations (the fundamentum inconcussum) of the truth of perception (Med 2 & 6, the clear and distinct primary qualities) and discursive inference (when it restrains itself to what is clear and distinct, Med 4). (Yes, according to this account, the ontolgocial argument at bottom can't be a discursive inference.)

Note, for example that in Meditation 2 one would think that clearly and distinctly perceiving that body but not mind can be doubted that he could finalize the "real" distinction of dualism there. But the real distinction is more than "I think. I exist." It does require inference (deducing it from the principle that what can be conceived as separable can be made by God as separable) and therefore can't be proven until Med 6.

The course of the meditations, then, is from (1) doubt to (2) certainty and truth restricted to cogito and God to (3) truths about the world and inferential truths, including those of logic.

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