Some brief and hurried thoughts on skepticism, based on Descartes' Meditations.
It seems impossible to be a true skeptic. Wasn’t this essentially what Descartes was trying to do using the Cartesian method of doubt (which employs methodological skepticism) in which he claims that he should doubt everything that is not completely certain and indubitable, because it might be false? Initially, he dismisses testimony of the senses for two reasons:
1. It is a mark of prudence to never place complete trust in those who have deceived us, even once and
2. Senses deceive us at night when we dream, so it follows that we could be dreaming at any time and be unaware that we are privy to such a state. So, he doubts sense testimony because it has deceived him in his dreams.
There are a number of obvious reasons to doubt Descartes’ total skepticism and the Cartesian method’s rationality:
1. Descartes doubts his senses because of “reason” and “prudence.” Aren’t these claims as open to doubt as any other claim Descartes considers? Where does reason suggest any such thing? If it did, one wouldn’t know where to begin, because it seems that practically everything we’ve ever thought can be subject to doubt! If we were to take Descartes claims seriously, we would be at a loss about what I could possibly believe at all, which is the perhaps self-defeating worry that skepticism seems to invite. If I’m to doubt everything, surely I should equally doubt the reasoning by which the skeptic leads me to take such doubts seriously!
2. We also have other reasons to think that Descartes’ claims about what reason and prudence recommend are in fact false:
a. Reason cares whether the possibilities being considered are genuine metaphysical ones arising in the context of the specific reasoning, or merely fictional, logical, or epistemic ones. Whether someone knows something or not depends a lot upon whether it’s a genuine possibility that they could be deceived in the circumstances.
b. Even allowing that many of one’s beliefs might be seriously dubitable, it would be foolish for reason to recommend withholding assent from all dubitable beliefs at once! It is one thing to doubt one thing at a time, which goes along the lines of fallibility principles (first person - you could be wrong, and general - everyone could be wrong), it’s quite another to doubt everything at once! Furthermore, there is no reason to believe this is even psychologically possible!
c. It is not clear that prudence advises us NEVER to place complete trust in anything (or person) who has deceived you even once. Ordinary prudence is a lot more intelligent – discriminating and selective – than that. For all the many of us that have been deceived in our dreams, we still completely trust our senses and memory at the moment regarding a zillion other things. It seems too much to require people to actually be able to justify everything they in fact know.
d. Doubting and thereupon asking for a justification of all one’s beliefs off hand would seem to be as unintelligible as asking in the case of space, not for the location of this or that object, but for the location of the whole of space. “Locating an object” ordinarily consists of locating it in relation to other objects whose location is known and taken for granted. The suggestion that we could somehow step outside the system and locate all of space is just playing with words, a fictional possibility. Being prepared to locate of justify anything does not entail being prepared to locate or justify everything at once. At any rate, until the skeptic has shown why we should attach even any sense to such a project, certainly he cannot claim that reason recommends we undertake it!
Here we are, at the beginning point in the Cartesian enterprise, in the position of the sea-surrounded mariner on Neurath’s boat. We must remain afloat while we repair little bits of it. Contrary to what Descartes presumably thought (in his dapper smoking jacket, warming himself by the fire - that's the image I like to entertain when I think about Descartes, anyway ;) ...), we can’t repair it all (be a total skeptic about everything) at once. Indeed, from the get-go Descartes’ doesn’t doubt what he thinks “reason” and “prudence” are, much less principles of mathematics and logic. A true skeptic wouldn’t even know where to start, because he would be forced to concede that principles of logic and mathematics could be deceptive and should be doubted as well. Utter skepticism about everything is impossible.