# Physical Probability

Three sub-topics:

1. The nature of physical probability
2. Inferring physical probabilities from symmetries
3. Probability coordination (how we get a handle on physical probability)

## The Nature of Physical Probability

More specifically, the nature of high-level, as opposed to irreducible (i.e., quantum) probability.

## Inferring Physical Probabilities from Symmetries

We humans are surprisingly good at figuring out the values of the physical probability distributions attached to a system from a rather small amount of information about the physical symmetries of the system. A simple example is our inference that the probability that a toss of a fair coin lands heads is one half. More interesting and useful inferences of this sort can be found in statistical physics and evolutionary biology.

In this work, I ask what method we use to make these inferences and why the method works so well. There is a positive and a negative part to the project. The negative part argues against the suggestion that we use a principle of indifference or some other ignorance-driven principle to make the inferences. The positive part attempts to give the correct account of the inferences.

I am now at work on a book, Tychomancy, devoted to the positive project. (The book is a major revision of an earlier long paper, Maxwell's Derivation of the Velocity Distribution: An Example of A Priori Discovery in Science?)

## Probability Coordination

A probability coordination principle instructs us, under certain circumstances, to set our subjective probability for an event equal to the physical probability for the event (and more generally, to set our subjective probability for an event conditional on a theory to the physical probability assigned to the event by the theory). Following these instructions appears to be the principal source of our grasp of the significance of probabilistic claims about the world. I have written on two kinds of questions about probability coordination: the question as to the proper form of the probability coordination principle, and the question as to why we are justified in conforming to the principle.