  Although philosophers have generally not been impressed by similar arguments for skepticism about the external world, there is little consensus on the status of arguments along the above lines. For example, Block writes: “I claim we simply do not know whether spectrum inversion obtains or not” (1990, 57). Argument C is less threatening than some other skeptical arguments because, even if sound, generalizations of it to skepticism about other kinds of mental states are implausible. For example, a parallel argument against one’s knowledge of other’s beliefs would have a dubious first premise.