ABSTRACT
Probabilities are easiest to define on finite sets. For example, consider a toss of a fair coin. Here “fair” means that heads and tails are equally likely. The situation may be represented by a set with two points H and T where H = “heads” and T = “tails.” The total probability of all possible outcomes is set equal to 1. Let “P(… )” denote “the probability of ….” If two possible outcomes cannot both happen, then one assumes that their probabilities add. Thus P(H) + P(T) = 1. By assumption P(H) = P(T), so P(H) = P(T) = 1/2.
