I’ve been on a David Lewis kick recently. Here’s more. David Lewis considers two different epistemic objections to Lewisian Modal Realism (hereafter LMR). The first argues that modal realism is incompatible with modal knowledge. The second is that modal realism is incompatible with knowledge about the external world. I want to talk about the second one.
Here’s the objection. If LMR is true, then there are as a matter of fact a very large number of persons that we can call “DECEIVEDs”. DECEIVEDs are your internal duplicates. The world appears to them exactly as it appears to you. But they are not in real worlds. They are being deceived. Some of them are deceived by evil demons, some of them are deceived by mad scientists, some of them are deceived simply in virtue of the fact that they are dreaming. If LMR is true, then you should be worried that you might be one of them.
Here’s one way to extract an argument against LMR, followed by a rough characterization of Lewis’ response.
A Road to Skepticism
- If LMR, then there are an incredibly large number of DECEIVEDs.
- If there are an incredibly large number of DECEIVEDs, then we cannot know that we are not one of the DECEIVEDs.
- If we cannot know that we’re not one of the DECEIVEDs, then we cannot know any proposition about the external world.
- Therefore, we cannot know any proposition about the external world.
Lewis’s Response: This is not a problem that is peculiar to LMR. The argument hinges on the fact that, for all you know, you could be one of these DECEIVEDs. Any view about modality that acknowledges it’s possible that you’re in one of these situations must grapple with this issue.
I think there they may be a difference here. Suppose I seem to see Ravens outside on the quad (and it’s true that there are Ravens). I think I know that there are Ravens. Now consider the following two cases.
Case One: You say, “How do you know there are ravens? Isn’t it possible that someone has filled the quad with fake ravens?”
Case Two: You say, “Don’t you know that there are teams of people all across the country that get a kick out of putting fake ravens on college campuses? They are not a small operation. They have fooled millions and millions of people”
I don’t think that the mere fact that someone alerts me to the possibility of deception undermines my knowledge/justification for the proposition that there are ravens on the quad. However, if someone alerts me to the fact that there are as a matter of fact lots and lots of fake ravens that have been planted on college campuses across the US. I think that does undermine my knowledge/justification (at a minimum it weakens it). So my intuitions that knowledge/justification are threatened is much stronger in Case Two.
On Lewis’ view of modality there really are loads of DECEIVEDs. So, on the face of it, his view resembles Case Two in a way that I think alternative views of modality do not.
Not sure that’s a great response to Lewis, but it seems like a way someone could try and resist Lewis’ response to the above argument. The idea is that other views about modality must deal with something like Case One. However, Lewis must deal with something like Case Two.
Thought I’d throw it out there.
Very interesting argument. The number of real, material DECEIVEDs really does seem to pose a more serious skeptical challenge than they would if they were mere abstract possibilities.
I agree with your assessment.
That is a cool point, but I am not sure I’m convinced that Lewis is in trouble. Consider the following case:
Case Three: Someone says, “Don’t you know that somewhere in the vast universe there are teams of people that get a kick out of putting fake ravens on college campuses? They are not a small operation. They have fooled millions and millions of people”
Now I am not sure that this new bit of information would undermine your knowledge/justification. After all, you don’t have much of a reason to believe that these pranksters are anywhere near your portion of the universe. I take it that Lewis could defend his view from skepticism by noting that the threat of skepticism is no more serious than the threat of lost knowledge given in Case Three. Aftera ll, you have no reason to believe that the evil demons, alien scientists or whatever are in your neck of the modal woods.
I was worrying about a similar point to that of Joshua’s: I think that our judgments regarding Case 1 might be underdetermined due to insufficient detail.
Suppose, for example, that we stipulate that in Case 1 the same group of people as in Case 2 are in fact operating in the area unbeknownst to both epistemic agents. In this example the possibility of error is just as close as it is in Case 2, although it is not salient to the conversation. The two cases seem to work out like Ginet/Goldman Fake Barn style skeptical scenarios with the following difference: In Case 1 the close possibility of error is unknown to the agents, but in Case 2 the close possibility of error is known to the agents (supposing that in Case 2 the ravens seen are in fact ravens).
Josh and Ezra,
Thanks for both of these points. I haven’t had a chance to reply because of end of the semester grading. I’ll be revisiting this soon once I get grades submitted.