Published online by Cambridge University Press: 01 January 2022
Rédei and Gyenis suggest that Lewis’s Principal Principle is meaningful only if it satisfies certain consistency conditions: starting from any assignment of credences to some algebra of events, we must always be able to extend our algebra with events as “the value of the objective chance of event E is p” and assign credences to such events in a consistent manner. I show that this extension is possible. However, I also argue that this requirement is unnecessary: the Principal Principle concerns subjective beliefs about objective chance; hence, events concerning those probabilities are meant to be in the algebra initially.
Special thanks are due to Miklós Rédei for introducing me to the issues surrounding the Principal Principle, for numerous discussions on the topic, and for inviting me to present this material at the Principal Principle Symposium of the 2014 PSA Biennial Meeting. I am indebted to Carl Hoefer as well for his valuable comments at the symposium. I would further like to express my gratitude for the discussions with Zalán Gyenis and with the Budapest-Krakow Research Group on Probability, Causality and Determinism.