 What does it mean to say that Sophia’s subjective probability for the proposition that tomorrow it will be sunny in Vienna equals .55? This is a difficult question. Let us first answer a different one. How do we measure Sophia’s subjective probabilities? On one traditional account, Sophia’s subjective probability for \(A\) is measured by her betting ratio for \(A\), i.e., the highest price she is willing to pay for a bet that returns $1 if \(A\), and $0 otherwise. On a slightly different account Sophia’s subjective probability for \(A\) is measured by her fair betting ratio for \(A\), i.e., that number \(r = b/(a + b)\) such that she considers the following bet to be fair: $\(a\) if \(A\), and $\(-b\) otherwise \((a, b \ge 0\) with inequality for at least one).