Probability, Probability Theory, Philosophy Of Mathematics.
possible outcomes for a two coin toss are HH, TT, HT, TH. Therefore, the probability of getting a HH or TT must sum to 0.5 (by additivity). So we want to represent P(HH ⋃ TT) = P(HH) + P(TT) = 0.5. Let X be the probability that my coin is a head and Y be the probability that your coin is a head.