Events and probability

Suppose an experiment has n equally likely, pairwise incompatible outcomes and m of them are favourable to event A. Then the probability of A is P(A) = m : n (the classical definition). The idea of probability took shape in the 17th century in the letters of Pascal and Fermat; the letter P comes from the Latin probabilitas. For a certain event m = n, so P = 1; for an impossible event m = 0, so P = 0. For a random event 0 < m < n, so 0 < P(A) < 1. Every probability lies between 0 and 1 and can be neither negative nor greater than 1.