William of Ockham - Wikipedia

Willam of Ockam was a great thinker of the Middle Ages. He is most famous for his contribution to science philosophy: The "Ockham's razor".
As a Franciscan monk, He also challenged the church authority and inspired the story of "The Name of the Rose".

Sailom

http://en.wikipedia.org/wiki/William_Ockham#Achievements

Achievements
Ockham has been called "the greatest nominalists that ever lived", and along with Duns Scotus, his opposite number from the realist camp, one of the two "greatest speculative minds of the middle ages", as well as "two of the profoundest metaphysicians that ever lived" (Peirce, 1869). One important contribution that he made to modern science and modern intellectual culture was through the principle of parsimony in explanation and theory building that came to be known as Ockham's razor. This maxim, as interpreted by Bertrand Russell (1946, 462—463), states that if one can explain a phenomenon without assuming this or that hypothetical entity, there is no ground for assuming it. That is, one should always opt for an explanation in terms of the fewest possible number of causes, factors, or variables.
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 sometimes considered an advocate of conceptualism rather than nominalism, for whereas nominalists held that universals were merely names, i.e. words rather than existing realities, conceptualists held that they were mental concepts, i.e. the names were names of concepts, which do exist, although only in the mind.
Ockham is also increasingly being recognized as an important contributor to the development of Western constitutional ideas, especially those of limited responsible government. The views on monarchial accountability espoused in his Dialogus* (written between 1332 and 1348) greatly influenced the Conciliar movement and assisted in the emergence of liberal democratic ideologies.
In mathematical logic Ockham worked towards what would later be called De Morgan's Laws and considered ternary logic - that is a logical system with three truth values, a concept that would become important in 20th century mathematics.
[edit]

Life

William of Ockham - Sketch labelled "frater Occham iste", from a manuscipt of Ockham's Summa Logicae, 1341
Ockham joined the Franciscan order while still very young and was educated first at the Franciscan house in London and then at Oxford. He did not complete his studies at Oxford, but it was during this period and the years immediately following that he wrote most of the philosophical and theological works on which his reputation primarily rests.
His ideas very soon became the subject of controversy. The earlier scholarly consensus that he was summoned to Avignon in 1324 by Pope John XXII on accusation of heresy, and spent four years there in effect under house arrest while his teaching and writing were investigated, has recently been challenged. He may in fact have been sent to Avignon in 1324 to teach philosophy at the prestigious Franciscan school, and made enemies among scholastic competitors, especially the followers of Thomas Aquinas (who had been canonized by John XXII one year before Ockham's arrival), some of whom accused Ockham of teaching heresy. There is evidence that it is not until 1327 that he was actually summoned before the Pope to answer charges made earlier by a commission of experts (without Franciscan representation), but no house arrest followed this exercise, with the Pope reserving judgement. Sometime after 9 April 1328, at the request of Brother Michael of Cesena, head of the Franciscan order, he investigated the controversy between the Franciscans and the Papacy on the doctrine of apostolic poverty, which had become central to Franciscan doctrine, but which was considered highly dubious and possibly heretical by both the Papacy and the Dominican order. He 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 Ockham's own philosophy, he fled Avignon on May 26, 1328 with Michael of Cesena and a few other friars. They eventually sought the protection of Emperor Louis IV of Bavaria. After his flight from the papal court, Ockham was excommunicated, but his philosophy was never officially condemned. He 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 became leader of the small band of Franciscan dissidents at Louis' court in 1342, after the death of Michael of Cesena.
He died on April 9, 1348 in the Franciscan convent in Munich, Bavaria (now Germany). He was posthumously rehabilitated by the official Church in 1359.