Probability theory is the branch of mathematics that deals with the possible outcomes of events and their relative likelihoods. While mathematicians agree on how to calculate the probability of certain events and how to use those calculations in certain ways, there's plenty of disagreement as to what the numbers actually mean. Probability divides into two main concepts: aleatory probability, which represents the likelihood of future events whose occurrence is governed by some random physical phenomenon like tossing dice or spinning a wheel; and epistemic probability, which represents our uncertainty of belief about past events that either did or didn't occur, or uncertainty about the causes of future events. The latter, for example, is what we're talking about when we say that it's "probable" that a certain suspect committed a crime based on the available evidence. It is an open question whether aleatory probability is reducible to epistemic probability based on our inability to precisely predict every force that might affect the roll of a die, or whether such uncertainties exist in the nature of reality itself, particularly at the level of quantum mechanics.