Full Idea
The three Kolgorov axioms of probability: the probability of an event is a non-negative real number; it is certain that one of the 'elementary events' will occur; and the unity of probabilities is the sum of probability of parts ('additivity').
Gist of Idea
Everything has a probability, something will happen, and probabilities add up
Source
PG (Db (ideas) [2031])
A Reaction
[My attempt to verbalise them; they are normally expressed in terms of set theory]. Got this from a talk handout, and Wikipedia.