Walter Burley

Walter Burley (or Burleigh; c. 1275 – 1344/45) was an English scholastic philosopher and logician with at least 50 works attributed to him. He studied under Thomas Wilton and received his Master of Arts degree in 1301, and was a fellow of Merton College, Oxford until about 1310. He then spent sixteen years in Paris, becoming a fellow of the Sorbonne by 1324, before spending 17 years as a clerical courtier in England and Avignon. Burley disagreed with William of Ockham on a number of points concerning logic and natural philosophy. He was known as the Doctor Planus and Perspicuus.

Early life
Burley was born in 1274 or 1275, possibly in Burley-in-Wharfedale, Yorkshire, or in Burley near Leeds. Little is known of his early life. He was made rector of Welbury in Yorkshire in 1309, probably through the influence of Sir John de Lisle, a friend of William Greenfield. As throughout his career, he did not act as rector, employing a substitute and using the income from the living to finance his study in Paris, where he completed his lectures on Peter Lombard's Sentences, and probably encountered the work of his contemporary William of Ockham. Burley's commentary on the Sentences has not survived.

Political career
Burley became a courtier during the political events that followed the deposition of Edward II of England in 1327. His first assignment was to try and obtain the canonisation of Thomas, 2nd Earl of Lancaster, who had been one of the leaders of the baronial opposition to Edward II; Thomas had become venerated as a martyr within a few months of his death. Burley was sent to the papal court at Avignon to appeal directly to Pope John XXII. By coincidence, William of Ockham was also staying at Avignon, having been summoned there in 1324 to answer charges of possibly heretical statements (by 1326 there was a list of 51 charges against him).

Burley's associates were all closely involved in these attempts at canonisation (none of which was successful). One was Richard de Bury, a bibliophile and patron of the arts and sciences, who became Burley's patron and at whose request Bury translated some works of Aristotle into English.

Philosophical work
Burley was one of the first medieval logicians to recognize the priority of the propositional calculus over the term logic, despite the fact that the latter had been the main focus of logicians until then. Burley also seems to have been the only 14th-century logician to have taken the position that, in line with modern views on the material conditional, the principle that "from the impossible anything follows" ("ex impossibili sequitur quodlibet") is both a necessary and sufficient condition for explaining the logical relationship between antecedent and consequent. He was also known for his commentaries on Aristotle's Physics, which include the quodlibet De Primo et ultimo instanti (around 1320) and the longer work Expositio in libros octo de physico auditu. Burley wrote 39 Commentaries on Aristotle and 32 Treatises and Questions (many unedited).

This is Burley's main work, in which he covers such topics as the truth conditions for complex sentences, both truth-functional and modal, as well as providing rules of inferences for different types of inferences. This book is known to have been written after Ockham's Summa Logicae (c. 1323), possibly partly in response to it. Other works include:
 * De Puritate Artis Logicae, in two versions:
 * Tractatus brevior (before 1324)
 * Tractatus longior (1325–28)
 * In Aristotelis Perihermenias (Questions on Aristotle's Perihermenias, 1301)
 * De consequentiis (1302)
 * De exclusivis (1302)
 * De exceptivis (1302)
 * De suppositions (1302)
 * De obligationibus (1302)
 * De ente (ca. 1310)
 * De qualitatibus (ca. 1310)
 * Expositio super libros De Anima (after 1310)
 * Tractatus de formis (1324-1326)
 * Tractatus de universalibus (after 1337)