I mean, you can invite the Cartesian Skeptic to a conversation, but if one conversant isn't willing to allow us to acquire reliable knowledge about the outside world, then really, there isn't much to talk about, and that would be a damned shame. And really it seems to me to be akin to the free will vs illusion of free will question. No matter which position is right, taking the negative position fails to yield anything useful, so it seems best to act as if we can have knowledge of the outside world and as if we have free will.