Perhaps surprisingly, Sosa's biggest objection to x-phi is that it hasn't gone far enough. The new field has revised philosophers' notions of what people actually think. But it's more important for philosophy to test whether they're right, he argues. Take skepticism, the theory that holds that we can't know anything for sure. Skepticism "relies crucially on the idea that for all we can really tell, life is but a dream," Sosa points out. This in turn depends on our common-sense conception of dreams as hallucinations. But is the dreaming brain really like the hallucinating brain? Why not enlist some neuroscientists to find out, Sosa says, and replace thought experiment with real experiment?
Other scholars argue that the changes wrought by experimental philosophy will in any case be limited. Some elemental principles of philosophy don't depend at all on assumptions about human psychology. And, when they do, philosophers can just replace the answers to a few questions with different ones (moral responsibility doesn't always require free will, for example) and then carry on as usual.
At the same time, certain recent experiments suggest that some philosophical questions will never be re-settled empirically, because the underlying psychology varies too widely among different groups of people. This fear has been voiced for years in philosophical debates on moral relativism, which contemporary philosophers have embraced less enthusiastically than other academics but haven't rejected entirely. And it's becoming increasingly inescapable.
Researchers from the United States and Brazil posed the following hypothetical: "A man goes to the supermarket once a week and buys a dead chicken. But before cooking the chicken, he has sexual intercourse with it. Is that wrong?" People in both countries said it's not OK to eat a sexed-up chicken considerably more often if they hailed from a low socioeconomic background. Cultural differences extend even to basic matters such as the meaning of language. Imagine that Gödel didn't invent Gödel's Theorem. Some guy named Schmidt did. Then to whom do we refer when we continue to use the word "Gödel"? In one experiment, researchers found that Americans tend to say, "the guy who got credit for the theorem," while Hong Kongers say, "the guy who actually came up with it."
Even if philosophers manage to put forth new theories based on answers that are replicated all over the globe, philosophy will never regain its old degree of certainty. What makes x-phi revolutionary, and horrifying to some, is that once philosophy opens up to the methods, and the irreducible uncertainties, of empirical science, its tenets can no longer be articles of faith. Philosophy is no longer something you believe in. It's something you test, and expect to change tomorrow.