 The notion of phenomenal indistinguishability of unrepeatable experiences cannot be defined as a statistical notion. Exactly how it should be defined is a matter of controversy. Gottlob Frege and Moritz Schlick raised doubts about the coherence of the notion of interpersonal phenomenal indistinguishability, (these ‘Frege-Schlick’ doubts are discussed by Stalnaker 2000, Shoemaker 1981 and 1996) In addition, though phenomenal indistinguishability is commonly taken to be a sort of phenomenal sameness, some philosophers use the expression to pick out a merely epistemic notion, involving experiences that subjects cannot distinguish (for discussion, see T. Williamson (1990), Martin (2004), Siegel (2004), and entry on  the disjunctive theory of perception.)  Martin and Williamson regard it as a substantive claim that experiences said to be ‘phenomenally the same’ have anything more than an epistemic property of indistinguishability in common.