The Sleeping Beauty paradox is a problem about conditional probability and self-locating belief. On Sunday, a fair coin
is tossed. If it lands heads, Sleeping Beauty is awakened on Monday only. If it lands
tails, she is awakened on Monday and Tuesday, with her memory of the Monday awakening
erased before Tuesday. Whenever she awakens, she is asked for the probability
that the coin landed heads (Elga 2000, Bischoff 2023).
"Halfers" answer , reasoning that awakening supplies
no new non-self-locating evidence about the fair coin (Lewis
2001). "Thirders" answer , treating the three possible
awakening situations--heads on Monday, tails on Monday, and tails on Tuesday--as
equally weighted from Beauty's perspective (Elga 2000). The disagreement concerns
how to update probabilities when an observer knows
that the observation protocol makes some outcomes produce more indistinguishable
observations than others. Closely related variants can have different answers if
the question, sampling procedure, or betting payoff is changed (Winkler 2017).
, reasoning that awakening supplies
no new non-self-locating evidence about the fair coin (Lewis
2001). "Thirders" answer 
, treating the three possible
awakening situations--heads on Monday, tails on Monday, and tails on Tuesday--as
equally weighted from Beauty's perspective (Elga 2000). The disagreement concerns
how to update probabilities when an observer knows
that the observation protocol makes some outcomes produce more indistinguishable
observations than others. Closely related variants can have different answers if
the question, sampling procedure, or betting payoff is changed (Winkler 2017).
