  Relatedly, McGinn (1989, 58–94) runs a “spatially Inverted Earth” argument against what he calls ‘strong externalism’ about perceptual content (a particular kind of externalism); Davies (1997) adapts McGinn’s example to argue for externalism about perceptual content. Both sides of the “externalism/internalism” debate as encapsulated in Argument F presuppose that it is possible to keep a perceiver’s internal constitution fixed while greatly varying the environment; this assumption is critically examined in Hurley 1998, Chs. 7, 8, and Myin 2001.