J M Keynes's foundation for his Principle of Indifference was a first order, predicate logic built on a propositional logic

In Chapter IV of A Treatise on Probability, John Maynard Keynes introduced the foundational elements of a first-order predicate logic that were needed to formalize and critically evaluate the Principle of Indifference (the rule assigning equal probabilities to alternatives when there is a symmetry of positive evidence or no reason to favor one alternative over another alternative).Instead of relying purely on a propositional framework, Keynes applied predicate-level distinctions to analyze the subjects, properties, and alternatives within an argument form. This allowed him to provide a set of restrictions and constraints on the application of the Principle of Indifference that would eliminate most of the contradictions, paradoxes and inconsistences that had plagued the Bernoulli-Laplace version of the rule that was originally called the Principle of Non-Sufficient Reason.