This note argues that a representation of the epistemic state of the individual through a non-additive measure provides a novel account of Keynes's view of probability theory proposed in his Treatise on Probability. The paper shows, first, that Keynes's "non-numerical probabilities" can be interpreted in terms of decisional weights and distorsions of the probability priors. Second, that the degree of non-additivity of the probability measure can account for the confidence in the assessment without any reference to a second order probability. And, third, that the criterion for decision making under uncertainty derived in the non-additive literature incorporates a measure of the degree of confidence in the probability assessment. The paper emphasises the Keynesian derivation of Ellsberg's analysis: the parallel between Keynes and Ellsberg is deemed to be significant since Ellsberg's insights represent the main starting point of the modern developments of decision theory under uncertainty and ambiguity.