Probability rules and conditional probability

In 1654 Antoine Gombaud, the Chevalier de Méré, a French writer and keen gambler, put a few gambling puzzles to Blaise Pascal. One was about dice: betting on at least one six in four throws seemed to pay, but betting on at least one double six in twenty-four throws of two dice seemed not to, although the two bets looked equivalent.

Pascal took the questions up in letters with Pierre de Fermat, and their correspondence is usually counted as the start of probability theory. It took until 1933 for Andrey Kolmogorov to give the subject the axioms every course now starts from.