dictum de omni et nullo also dici de omni et nullo (Latin, ‘said of all and none’), two principles that were supposed by medieval logicians to underlie all valid syllogisms. Dictum de omni applies most naturally to universal affirmative propositions, maintaining that in such a proposition, whatever falls under the subject term also falls under the predicate term. Thus, in ‘Every whale is a mammal’, whatever is included under ‘whale’ is included under ‘mammal’. Dictum de nullo applies to universal negative propositions, such as ‘No whale is a lizard’, maintaining that whatever falls under the subject term does not fall under the predicate term. See also SYLLOGIS. W.E.M.