*Probability from Symmetry*

*Probability from Symmetry*### Ezra Rubenstein, Berkeley

**Abstract: **At the heart of the ‘classical’ approach to probability is the idea that the probability of a proposition quantifies the proportion of possible worlds in which it is true. This idea has a proud history, but it has fallen on hard times. I aim to rejuvenate it. Firstly, I show how the metaphysics of quantity together with the physical laws might yield well-defined relative sizes for sets of physically possible worlds. Secondly, I argue that these relative sizes are apt to reconcile the credence-guiding and frequency-explaining roles of probability.