Axiomatic Approach
Axiomatic approach is a way of describing probability of an event. If S is the sample space of a random experiment and E is the power set of S, then a real valued function from E to [0,1] is probability function if it satisfies the following axioms:

Axiom of Positivity
P(E) ≥ 0 if E ∈ f(s)

Axiom of Certainty
P(S) = 1, when S is the sample space

Axiom of Union
P(A∪B) = P(A) + P(B) for every exclusive events A and B