Ballarin2017
Ballarin, Roberta, "Modern Origins of Modal Logic", The Stanford Encyclopedia of Philosophy (Summer 2017 Edition), Edward N. Zalta (ed.), URL: https://plato.stanford.edu/archives/sum2017/entries/logic-modal-origins/
Ballarin, Roberta, "Modern Origins of Modal Logic", The Stanford Encyclopedia of Philosophy (Summer 2017 Edition), Edward N. Zalta (ed.), URL: https://plato.stanford.edu/archives/sum2017/entries/logic-modal-origins/