Axioms of probability

Kolmogorov's axioms of probability can be stated in terms of the sample space S of possible events, E1, E2, E3, …En and the real-valued probability P(E) of an event E. The axioms are:

1. P(E) ≥ 0 for all E ϵ S
2. P(S) = 1
3. 

Together, these axioms say that probabilities cannot be negative numbers—impossible events have zero probability—no events outside the sample space are possible as it is the universe of possibilities under consideration, and that the probability of either of two mutually exclusive events occurring is equal to the sum of their individual probabilities.