William of Ockham
William of Ockham (also Occam or any of several other spellings) (ca. 1285-1349) was a Franciscan friar and philosopher, from Ockham (near Ripley, Surrey), England. William devoted to a life to extreme poverty and minimalism. A pioneer of nominalism, some consider him the father of modern epistemology and modern philosophy in general, because of his strongly argued position that only individuals exist, rather than supra-individual universals, essences, or forms, and that universals are the products of abstraction from individuals by the human mind and have no extra-mental existence. Ockham is also considered one of the greatest logicians of all time.
Dave Beckett of the University of Kent at Canterbury writes:
- "The medieval rule of parsimony, or principle of economy, frequently used by Ockham came to be known as Ockham's razor." 
Summoned to Avignon
by Pope John XXII
on accusation of heresy, William spent four years there in effect under house arrest while his teaching and writing were being investigated. During this period, at the request of Brother Michael of Cesena
, head of the Franciscan
order, Ockham investigated the controversy between the Franciscans and the Papacy on the doctrine of apostolic poverty
, which was central to Franciscan doctrine but anathema to the Pope. Ockham concluded that Pope John XXII was a heretic, a position that he later put forth in writing. Before a conclusion was reached about the heresy or orthodoxy of William's philosophy, he fled Avignon on May 26
with Michael of Cesena and a few other friars. They sought the protection of Emperor Louis IV in Bavaria. 
After criticizing the pope, he is believed to have been excommunicated, although historical sources vary. Ockham spent much of the remainder of his life writing about political issues, including the relative authority and rights of the spiritual and temporal powers.
He died in a convent in Munich, Bavaria (now Germany).
- Summa logicae (Sum of Logic) (before 1327), Paris 1448, Bologna 1498, Venice 1508, Oxford 1675.
- Quaestiones in octo libros physicorum, (before 1327), Rome 1637.
- Summulae in octo libros physicorum, (before 1327), Venice 1506.
- Quodlibeta septem (before 1327), Paris 1487.
- Expositio aurea super totam artem veterem: quaestiones in quattuor libros sententiarum, Lyons 1495, Bologna 1496.
- Major summa logices, Venice 1521
- Quaestiones in quattuor libros sententiarum, Lyons, 1495.
- Centilogium theologicum, Lyons 1495.
- Questiones earumque decisiones, Lyons 1483.
- Quodlibeta septem, Paris 1487, Strassburg 1491.
- Centilogium, Lyons 1494.
- De sacramento altaris and De corpore christi, Strassburg 1491, Venice 1516.
- Tractatus de sacramento allans
- Opus nonaginta dierum (1330-1332), Louvain 1481, Lyons 1495.
- Dialogus…de imperatorum et pontificum potestate, Lyons 1495.
- Compendium errorum Joannis XXII papae (1335-1338), Paris 1476, Lyons 1495.
- Defensorium contra errores Johannem XXII papae (1335-1339) Venice 1513.
- Super potestate summi pontificis octo quaeslionum decisiones (1339-1342).
- Tractatus de dogmatibus Johannis XXII papae (1333-34).
- Adversus errores Johannis XXII, Louvain 1481, Lyons 1495.
- Epistola defensoria, Venice 1513.
- Decisiones octo quæstionum (after 1339), Lyons 1496.
- Dialogus in tres partes diatinctus (1342-43), Paris 1476.
- De jurisdictione imperatoris in causis matrimonialibus, Heidelberg 1598.
- De electione Caroli IV (last work)