  Kripke first became interested in modal logic in 1956, as a result of reading Prior’s paper ‘Modality and Quantification in S5’ (Prior 1956a). Kripke was at this time still at high school, working on logic in almost complete isolation in Omaha, Nebraska. In 1958, he read Time and Modality and was impressed by the parallel Prior drew between tense and the alethic modalities.