Twin-earth externalism and concept possession

It is widely believed that Twin-Earth-style thought experiments show that the contents of a person's thoughts fail to supervene on her intrinsic properties. Several recent philosophers have made the further claim that Twin-Earth-style thought experiments produce metaphysically necessary conditions for the possession of certain concepts. I argue that the latter view is false, and produce counterexamples to several proposed conditions. My thesis is of particular interest because it undermines some attempts to show that externalism is incompatible with privileged access.