Ludwig Wittgenstein (1889 - 1951)

Considered by some to be the greatest philosopher of the 20th century, Ludwig Wittgenstein played a central, if controversial, role in 20th-century analytic philosophy. He continues to influence current philosophical thought in topics as diverse as logic and language, perception and intention, ethics and religion, aesthetics and culture. 

There are two commonly recognized stages of Wittgenstein's thought — the early and the later — both of which are taken to be pivotal in their respective periods. 

The early Wittgenstein is epitomized in his Tractatus Logico-Philosophicus. By showing the application of modern logic to metaphysics, via language, he provided new insights into the relations between world, thought and language and thereby into the nature of philosophy. 

It is the later Wittgenstein, mostly recognized in the Philosophical Investigations, who took the more revolutionary step in critiquing all of traditional philosophy including its climax in his own early work. The nature of his new philosophy is heralded as anti-systematic through and through, yet still conducive to genuine philosophical understanding of traditional problems.

William Ockham (1287 - 1347)

William of Ockham is, along with Thomas Aquinas and John Duns Scotus, among the most prominent figures in the history of philosophy during the High Middle Ages. He is probably best known today for his espousal of metaphysical nominalism; indeed, the methodological principle known as “Ockham's Razor” is named after him. But Ockham held important, often influential views not only in metaphysics but also in all other major areas of medieval philosophy—logic, physics or natural philosophy, theory of knowledge, ethics, and political philosophy—as well as in theology.

Ockham's is not much concerned with answering skeptical doubts. He takes it for granted that humans not only can but frequently do know things, and focuses his attention instead on the “mechanisms” by which this knowledge comes about.  Ockham's theory of knowledge, like his natural philosophy, is broadly Aristotelian in form, although—again, like his natural philosophy—it is “Aristotelian” in its own way. For most Aristotelians of the day, knowledge involved the transmission of a “species” between the object and the mind. At the sensory level, this species may be compared to the more recent notion of a sense “impression.”