Cartesian skepticism has been a hot topic lately at TSZ. I’ve been defending a version of it that I’ve summarized as follows:
Any knowledge claim based on the veridicality of our senses is illegitimate, because we can’t know that our senses are veridical.
This means that even things that seem obvious — that there is a computer monitor in front of me as I write this, for instance — aren’t certain. Besides not being certain, we can’t even claim to know them, and that remains true even when we use a standard of knowledge that allows for some uncertainty. (There’s more — a lot more — on this in earlier threads.)
In explaining to Kantian Naturalist why I am a Cartesian skeptic, I introduced the analogy of the Sentinel Islander:
Here’s an analogy that shows how serious the problem of circularity is for your position.
Suppose that a few decades from now you possess a really high-fi pair of virtual reality goggles, plus some sensitive motion sensors. You kidnap a North Sentinel Islander who knows nothing about virtual reality or computers, and you tell him that the goggles and sensors are magical devices that can grant him access to an actual land, LaLa Land, which is far away.
The islander learns to navigate LaLa Land successfully, even carrying out tasks within it. If you ask him questions about LaLa Land, he answers them “correctly”. He even claims to know things about LaLa Land, which he takes to be real. We know better, because we understand that the goggles do not deliver veridical sensory information. They are fostering an illusion. LaLa Land doesn’t exist in the real world.
The islander could argue, KN-style:
1. I assume that the goggles deliver veridical information about LaLa Land.
2. On the basis of that assumption, I am able to navigate LaLa Land successfully and satisfy my goals.
3. Therefore, the goggles deliver veridical information about LaLa Land.
Is he right? Obviously not. We can see that he is being fooled, and we can diagnose the problem with his argument: it’s blatantly circular.
How is your argument any better than his?
If we assume that our perceptions are veridical, we are making an equivalent mistake to the Sentinel Islander when he assumes that the VR goggles deliver veridical information about LaLa Land.
On the other hand, if we don’t know that our perceptions are veridical, and we can’t even judge the likelihood that they are veridical, then we are in no position to claim knowledge — justified true belief — regarding the external world.