A Nightmare for Lewisian Halfers

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, the credence 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. I also respond to and improve upon a new type of defence of Lewisian halfing by Schwarz (2025), but argue that it fails as well.