The philosophy of probability pioneered by Ramsey and de Finetti, and furthered by the American statistician L. J. Savage in his Foundations of Statistics (1954). Personalism rejects the view that probabilities are ‘out there’ waiting to be discovered. It views assignments of probability to events as purely personal expressions of the degree of confidence to be had in the occurrence of an event. Saying that an event is 50% probable is simply an expression or endorsement of a strategy of betting on the event at evens. Betting rates must, however, be coherent in the sense of conforming to the mathematics of probability, since if an agent buys and sells bets without obeying its laws (for example, assigning a high probability both to an event e, and to its complement, not-e) he can be put in the position of losing whatever the outcome. This constraint is known as that of avoiding a Dutch book. The convergence of arbitrarily different initial betting rates upon a stable (‘objective’) probability is due to the pressure of evidence, which forces an agent who seeks coherence through time to conditionalize (see conditional probability), or modify his probability assignments in accordance with some version of Bayes’s theorem. See also exchangeability, representation theorem, principal principle.

In an older usage, personalism is the theistic stress on the existence of divine personality, or any philosophy according to which the individual person or thinker is the starting-point of theory.