He is considered - along with Thomas Aquinas, Duns Scotus, and the Islamic scholar Averroes - to be one of the major figures of medieval thought and was at the centre of the major intellectual and political controversies of the fourteenth century. Although commonly known for Occam's razor, the methodological principle that bears his name, William of Ockham also produced significant works on logic, physics, and theology. In the Church of England, his day of commemoration is April 10.
In scholasticism, Ockham advocated a reform both in method and in content, the aim of which was simplification. Ockham incorporated much of the work of some previous theologians, especially John Duns Scotus. From Scotus, Ockham derived his view of divine omnipotence, his view of grace and justification, much of his epistemology and ethical convictions. However, he also reacted to and against Scotus in the areas of predestination, penance, his understanding of universals, his distinction ex parte rei (that is, "as applied to created things"), and his view of parsimony.
A pioneer of nominalism, some consider him the father of modern epistemology, 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.[8] He denied the real existence of metaphysical universals and advocated the reduction of ontology.
Ockham wrote a great deal on natural philosophy, including a long commentary on Aristotle's physics. According to the principle of ontological parsimony, he holds that we do not need to allow entities in all ten of Aristotle's categories; we thus do not need the category of quantity, as the mathematical entities are not "real". Mathematics must be applied to other categories, such as the categories of substance or qualities, thus anticipating modern scientific renaissance while violating Aristotelian prohibition of metabasis.
In the theory of knowledge, Ockham rejected the scholastic theory of species, as unnecessary and not supported by experience, in favour of a theory of abstraction. This was an important development in late medieval epistemology. He also distinguished between intuitive and abstract cognition; intuitive cognition depends on the existence or non existence of the object, whereas abstractive cognition "abstracts" the object from the existence predicate. It is not yet decided among interpreters as to the role of these two types of cognitive activities.