 The concept of fuzzy sets was developed in the mid-twentieth century at Berkeley  as a response to the lacking of probability theory for jointly modelling uncertainty and vagueness.Bart Kosko claims in Fuzziness vs. Probability that probability theory is a subtheory of fuzzy logic, as questions of degrees of belief in mutually-exclusive set membership in probability theory can be represented as certain cases of non-mutually-exclusive graded membership in fuzzy theory.In that context, he also derives Bayes' theorem from the concept of fuzzy subsethood.