Probability theory is the set of laws underlying rational belief. The mathematics of probability describes equally and without distinction (a) figuring out where your bookcase is, (b) figuring out the temperature of the Earth’s core, and (c) estimating how many hairs were on Julius Caesar’s head. It’s all the same problem of how to process the evidence and observations to revise (“update”) one’s beliefs. Similarly, decision theory is the set of laws underlying rational action, and is equally applicable regardless of what one’s goals and available options are.
