Phenomenal conservatism is the following claim:
PC: If it seems to S that p, then S is prima facie justified in believing p.
The stronger the seeming, the greater the prima facie justification that is provided by it. However, the justification is only prima facie justification – it is subject to defeat. This justification can be defeated when S is more justified in believing not-p (perhaps because it more strongly seems to S that not-p), or when the justification provided by that seeming is undercut (perhaps because it seems to S that a seeming about such a matter is unreliable or misleading).
So, what’s the problem? Suppose all the things that Charlie Sheen has been saying are things that actually seem true to him. If so, then according to PC, Sheen has prima facie justification for believing them. That itself is not a problem, since this justification can be defeated. The problem is that it doesn’t seem to be. We can assume that the opposite of what Sheen is claiming does not seem true to him, and it doesn’t seem like Sheen has an undercutting defeater either. Discovering those that disagree can do this, but it also seems to Sheen that he cannot be fathomed by a human mind, so according to PC he has prima facie justification for believing that all (other?) humans cannot give him reason to think that he is malfunctioning. We can also suppose that it seems to Sheen that he is operating just fine.
So, it looks like PC has it that Sheen is ultima facie justified in believing the things he has been claiming – that he is on balance justified in believing them. Many find that to be an absurd consequence of PC. Is it?
I don’t think so. For one, I think that you can be justified in believing just about anything, so that a view allows you to be so, is an asset. In this case, we are imagining that Sheen has massively misleading evidence, that’s unfortunate for him, but that is a way things can be. It just may be that Sheen is believing exactly as he should given his evidence – given the way things seem to him.