The probability of an event is:


This assumes that all outcomes are equally likely and that you’ve got a finite sample space.

This works if, for example, you think about a fair coin or dice roll.

However, the definition breaks down outside of the most trivial examples. Intuitively, the probability of aliens existing on the moon isn’t (either they do, or they don’t); nor should the probability of blue aliens shaped specifically like a giraffe be equal to the probability of any aliens in general existing on the moon…it would be much less.