Bayesianism is the opinion that probabilities are a matter of opinion. This is because they would only measure the strength of our beliefs (De Finetti 1972; Jeffreys 1975; Keynes 1957; Savage 1954). That view is generally known as Bayesianism because of its heavy reliance on a certain interpretation of Bayes' theorem, a piece of pure mathematics that actually refers neither to the knowing subject nor to the real world.