Response from Peter Lipton on November 2, 2005

In his Meditations, Descartes thought he found such a circumstance, namely his belief that he himself exists (as a 'thinking thing'). For he couldn't even imagine anything unless he existed.

This is a beautiful line, taking us not from our thoughts out to the external world that seems to cause them but from our thoughts in to the mind that must exist in order for us to have them. But pesky philosophers have raised numerous difficulties with Descartes strategy. I'll mention just two.

The first is that even if Descartes Cogito ergo Sum maneuver works, it is hard to see how to extend it to show that we know anything apart from the existence of our own minds.

The second is that Descartes doesn't really even seem entitled to belief in the existence of his own mind, if that is something with a past or a future, indeed if it is something that is distinct from the thought itself. In the end, the promising 'I think therefore I am' seems to reduce to the less impressive 'There is a thought now therefore there is a thought now'. If that.

Response from Nicholas D. Smith on November 3, 2005

There is yet another way to doubt what Descartes supposed we could know with certainty. It seems that one might also wonder if we might be fallible even in the ways in which we categorize our own experiences. Descartes imagines a very powerful malin genie (an evil demon) who would delight in confusing us about anything and everything. Couldn't such a being even confuse us about how to categorize experiences as thoughts (rather than, say, headaches or tickles). To identify something as a thought (and not a headache or a tickle), we have to be able to rely on the conceptual apparatus by which we make such distinctions. It is not obvious that our conceptual apparatus on this or any other subject is invulnerable to error or confusion--especially given the best efforts of some evil divinity!