Origin of probabilities

Probabilities are normally assumed to enter for at least two reasons: (a) because of random variation between individuals, and (b) because of random errors in measurement. (a) is particularly common in the biological and social sciences; (b) in the physical sciences. In physics effects of statistical mechanics and quantum mechanics are also assumed to introduce probabilities. Probabilistic models for abstract mathematical systems have in the past been rare, though the results about randomness in this book may make them more common in the future.