Published online by Cambridge University Press: 01 January 2020
Consider a language SL having as its primitive signs one or more atomic statements, the two connectives ‘∼’ and ‘&,’ and the two parentheses ‘(’ and ‘)’; and presume the extra connectives ‘V’ and ‘≡’ defined in the customary manner. With the statements of SL substituting for sets, and the three connectives ‘∼,’ ‘&,’and ‘V’ substituting for the complementation, intersection, and union signs, the constraints that Kolmogorov places in [1] on (unary) probability functions come to read:
K1. 0 ≤ P(A),
K2. P(∼(A & ∼A)) = 1,
K3. If ⊦ ∼(A & B), then P(A ∨ B) = P(A) + P(B),
K4. If ⊦ A ≡ B, then P(A) = P(B).2