 Walter Burley, or Burleigh, (ca. 1275–1344) was one of the most prominent and influential philosophers of the fourteenth century. He had a very long career in both England and France, becoming Master of Arts at Oxford by 1301 and Master of Theology at Paris by 1324. He produced a large body of about fifty works, many of which were widely read in the later Middle Ages. Especially prominent were his last commentaries on the Ars Vetus and Physics, which were studied all over Europe and particularly at Italian universities during the latter half of the fourteenth and the whole of the fifteenth centuries. His semantic and ontological views evolved during his career in response to Ockham’s acute criticism of traditional realism, moving from the moderate realism typical of thirteenth-century theologians such as Thomas Aquinas and Henry of Ghent to the extreme realism of his later writings, which posit the existence of extramental universals really distinct from individual things, extramental propositions as significates of true sentences, and real distinctions among the ten categories. According to Burley, all of this is necessary to preserve the validity of our knowledge of the external world, which he believed can be made evident without falling prey to Ockham’s criticisms.