Ackermans, Lennart B. (2026) A Nightmare for Lewisian Halfers. [Preprint]

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Abstract

What if Sleeping Beauty has dreams? I give an argument against the halfer position that only uses principles of rationality accepted by Lewisian halfers. While dreaming, Beauty’s credence in heads is 1/2. After being woken up and updating on the evidence that she is woken up today, it becomes less than 1/2. For Lewisian halfers, this is a nightmare. If we add two plausible assumptions about Beauty’s dreaming credences, we can derive the thirder solution.

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Item Type:	Preprint
Creators:	Creators Email ORCID Ackermans, Lennart B. philosophy@ackermans.ch 0000-0003-0071-2270
Keywords:	Sleeping Beauty, Lewisian halfism, self-locating evidence, Bayesian conditionalization, expected accuracy
Subjects:	General Issues > Decision Theory Specific Sciences > Probability/Statistics
Depositing User:	Lennart Ackermans
Date Deposited:	18 Jan 2026 21:51
Last Modified:	18 Jan 2026 21:51
Item ID:	27940
Subjects:	General Issues > Decision Theory Specific Sciences > Probability/Statistics
Date:	17 January 2026
URI:	https://philsci-archive.pitt.edu/id/eprint/27940

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