  If a behaviorally undetectable spectrum inversion scenario is possible, how do we know it does not actually obtain? There is a temptation to say that we do not know—moreover, that we cannot know. (For more discussion, see subsection 3.6 below.) And if we cannot verify or falsify the claim that such a scenario does not obtain, then according to the verificationist theory of meaning held by the logical positivists (Ayer 1959) the scenario is meaningless. Indeed, the logical positivists sometimes gave spectrum inversion as an example of an unverifiable claim (Schlick 1932/3, 93, quoted in Shoemaker 1982, 339; see also Ayer 1936, 173–4, and Wittgenstein 1958, §272).