In case an experiment has finitely many outcomes and all outcomes are equally likely
to occur, the probability of an event (set of outcomes) is just the ratio of the number
of outcomes in the event to the total number of outcomes. Strictly speaking, this result
can be deduced from a set of axioms for probability formulated in 1933 by the Russian
mathematician A. N. Kolmogorov. In Section 9.8 we discuss the axioms and show how to
derive their consequences formally. At present, we take a naïve approach to probability
and simply state the result as a principle.