Descartes' arguments suggest that he believe d that, for any empirical test that you might devise to determine whether you are awake or dreaming, it might be the case that anytime you appeal to putative test results you have merely dreamed that you have performed the test. So, I think you are right that, whatever arguments Descartes developed to respond to his own skeptical doubts, those arguments were not straightforward empirical ones appealing to things like tests of that sort. Similarly, when he addressed issues related to skepticism Descartes' eighteenth-century successor developed a style of non-empirical transcendental argumentation. Another way to approach the issues that your question engages is to focus not on Descartes' specific argument but rather on the general skeptical doubt that it can raise so hauntingly. The contemporary philosophy Barry Stroud puts this doubt this way in his wonderful book, _The Significance of Philosophical Skepticism:"Could not the external world be completely...

As Richard suggests, the success or failure of arguments of this sort depends on the success or failure of arguments about the nature of the content of our beliefs and other thoughts. So, for example, in his later writings Davidson made clear that his account of (as he called it) the veridicality of belief--his account of why by their very nature the vast majority of our beliefs must be true--depends on his defense of a version of a doctrine that philosophers call "externalism," and which asserts that the content of our beliefs and other thoughts is (in part) determined by the very external objects those thoughts purport to be about. If this view of the content of our beliefs is correct, then the thought that the world might be completely different from what we perceive it and believe it to be would be mistaken in a rather strong sense: despite appearances, massive error of that would turn out to be unintelligible. Understanding whether or not any one view of content is correct is a...

I agree that acknowledging that "the mere possibility of doubt doesn't provide a reason for doubting" is one part of a sensible answer to your question: that (1) we aren't sure whether we can prove complete certainty that your dog--or that Allen's scrap of tin-foil--does not control the world, despite all appearances to the contrary, doesn't provide us with (2) a good reason for taking seriously what certainly looks to be an extremely remote possibility. Moreover, and as the reference to tin-foil suggests, (3) there seems something irrational about taking those doubts too seriously. But why do possibilities like these seem easy to ignore or even to dismiss as "crazy thoughts"? Is this just because we are unwilling to challenge received opinion? Is this dismissive stance yet another example of the human tendency to embrace dogmatism? I believe is what a student of mine had in mind as she stormed out of the classroom when I offered this sort of response to similar doubts that she had raised. I...

As Saul's response makes clear, Descartes' own reasoning seems to rely heavily on his argument for the existence of God. I think that few today would accept that argument, and so although studying Descartes closely would doubtless be interesting it may not give you a satisfactory answer to your questions. Another form of argumentation that purports to be capable of proving the existence of objects distinct from oneself is transcendental argumentation. This is most closely associated with Immanuel Kant, who developed his famous transcendental argumentation (at least in part) because he was interested in responding to problems with the rationalist philosophical tradition in which Descartes wrote and which was critiqued severely by David Hume. As interesting as Kant's argumentation is--and as fascinating as is an historical account of Kant's responses to his illustrious predecessors--Kant's own argumentation may not help you to find an answer to your question: although Kant answered your...

The compatibility or incompatibility of divine omniscience and mortal freedom interests me a lot, although the concept of the "author of one's own nature" strikes me as relatively unclear and probably not that useful for investigating this. Sean sketches out one answer that may be satisfactory to those who believe that free will is compatible with all of one's choices being determined by preceding events. I'm not sympathetic toward this sort of "compatibilism," and I've never been persuaded by any argument that free will could exist a world where there could be an omniscient God. So, my short answer to your question is that I think that omniscience does negate free will. (My own position is an unpopular one, and as Sean suggests a lot depends on exactly how you define the key terms used in the original question and in the answer that I just gave....)