Von Kries and the other ‘German logicians’: non-numerical probabilities before Keynes

Keynes's A Treatise on Probability (Keynes, 1921) contains some quite unusual concepts, such as non-numerical probabilities and the ‘weights of the arguments’ that support probability judgements. Their controversial interpretation gave rise to a huge literature about ‘what Keynes really did mean’, also because Keynes's later views in macroeconomics ultimately rest on his ideas on uncertainty and expectations formation.