Introduction

William of Ockham was born around 1285, probably in the village of Ockham near London. He entered the Franciscan order and studied at Oxford. There he completed the requirements for a master of theology degree but never became a regent master, at least in part because the Chancellor of the University, John Lutterell, opposed his appointment. In 1323 he was accused of heresy by Lutterell and went to Avignon the following year to answer charges. He spent four years in Avignon and ended by becoming involved in the Franciscan quarrel over poverty. With other Franciscans he left Avignon in 1328 and was subsequently excommunicated. Louis of Bavaria protected the fugitive Franciscans, and Ockham came to reside with Louis at Munich, where he wrote political treatises against the pope. He died in Munich in 1347 during an outbreak of the black plague. Besides his numerous political treatises, he also wrote commentaries on Aristotle and on the Sentences of Peter Lombard, as well as various works on logic. The following selection is taken from his major logic book, Summa logicae, which was probably written before 1324, while he was still in England.

Well into the thirteenth century modal logic was heavily dependent on Aristotle's understanding of modality. Aristotle's modal notions can be thought of as ways of classifying what happens in the actual world at different moments of time, and his modal logic is characterized by the principle of plenitude: No genuine possibility can remain forever unactualized.