John Greco joins Ken and John in their discussion of Skepticism. Greco states that one of the most radical forms of skepticism is one of the most deep and fundamental kinds of skepticism. Greco thinks that what's interesting about skepticism is not that there are strictly philosophically skeptical people - it's the arguments that start from innocent premises and reach astounding conclusions. Ken asks for an example.

John Greco refers to Descartes' Meditations about one such argument. The gist of Descartes argument is that the kind of evidence we have for our beliefs underdetermines what to believe. To support this point, John gives Bertrand Russell's example. Suppose you had a series of dreams that picked up from where it let off the previous night. In such a situation, how could you distinguish your 'dream' life from your 'real' life? Greco states the subtle point that the skeptic never actually claims that we are really dreaming instead of living. The skeptic claims that our current evidence does not rule out the possibility that we may actually be dreaming.

John explains that there are alternative explanations that describe a given situation. If you find two kids in a room and one has his mouth smeared with chocolate, one explanation is that the kid who has chocolate smeared on his mouth ate the cookie. Another is that the kid with the clean mouth ate it and smeared the cookie on the mouth of his friend to deflect the blame. John asks how we block these alternative explanations. Greco expands on this point by saying that skeptic starts from very innocent, intuitive points (such as the multiple explanations for a single situation) and arrives at clearly wrong conclusions. He likens the skeptic's arguments to Zeno's paradox that there is no motion. Clearly, Zeno's argument has gone wrong somewhere. Similarly, clearly, the skeptic must have gone wrong somewhere.

John thinks that skepticism can lead to a fruitful inquiry in the following way. Take the Sorites Paradox. First, accept these three premises. If you have a head mostly full of hair, then you are not bald. And if you have mostly no hair then, you are not bald. Also, if you take one hair from your head, this doesn't make you bald. So keep taking one hair from a hairy head. According to our premises, you shouldn't get a bald head. However, you would get a bald head. John explains that important logic and mathematics have arisen thousands of pages that deal with this paradox. Similarly, John thinks that skepticism can lead to a similar sort of accomplishments.

Ken explains that idealism was one of the proposed solutions to skepticism. Skepticism makes a distinction between our ideas or perceptions and things that give rise to these ideas and perceptions such as dreams or the normal wakeful life experience. Idealism by claiming that the world is just our ideas proposes to dissolve the problem of skepticism. For instance, there is nothing over and above the existence of a table than just the sensation that there is a table there. Greco thinks that philosophers have adopted such extreme positions is a testament to the strength of skeptical arguments. However, idealism ultimately is perhaps more counterintuitive than skepticism and ought to be rejected on the basis of commonsense.

Ken suggests that skeptical arguments may be ultimately successful. He suggests that we give up the concept of knowledge for something like "reasonable to believe." I don't know if I am in a brain in a vat but I believe I am not and it is reasonable to believe so. Do I know? No, but Ken thinks that is not so crucial. Greco replies that such a move would assume that skeptical arguments depend on a particularly strong conception of knowledge. Greco believes that best skeptical arguments don't depend on such a strong conception.

• The Roving Philosophical Reporter (Seek to 4:50): Polly Stryker interviews Michael Shermer, the director of Skeptic Society.
• The 60-second philosopher (Seek to 49:47): Ian Schoales philosophizes rapidly about different strands of skepticism.