1. P(a) should be a number between 0 and 1.

2. If a represents a certain event then P(a)=1.

3. If a and b are mutually exclusive events then P(a or b) = P(a) + P(b).

(mutually exclusive means that they cannot both be true at the same time; for example, if a represents the proposition that our control system contains 0 fault, while b represents the proposition that out control system contains 1 fault)