But hang on, the foundation of Bayesianism is the counterfactual. P(A|B) = 0.6 means that "If B were true, then P(A) = 0.6 would be true". Where does the truth value of P(A) = 0.6 come from then if we are to accept Bayesianism as correct?

But hang on, the foundation of Bayesianism is the counterfactual. P(A|B) = 0.6 means that "If B were true, then P(A) = 0.6 would be true". Where does the truth value of P(A) = 0.6 come from then if we are to accept Bayesianism as correct?