H. P. Grice, "Heytesbury's Implicature"

Heytesbury, William, also called Hentisberus, Hentisberi, Tisberi (before 1313–c.1372), English philosopher and chancellor of Oxford University. He wrote Sophismata (“Sophisms”), Regulae solvendi sophismata (“Rules for Solving Sophisms”), and De sensu composito et diviso (“On the Composite and Divided Sense”). Other works are doubtfully attributed to him. Heytesbury belonged to the generation immediately after Thomas Bradwardine and Kilvington, and was among the most significant members of the Oxford Calculators, important in the early developemnt of physics. Unlike Kilvington but like Bradwardine, he appealed to mathematical calculations in addition to logical and conceptual analysis in the treatment of change, motion, acceleration, and other physical notions. His Regulae includes perhaps the most influential treatment of the liar paradox in the Middle Ages. Heytesbury’s work makes widespread use of “imaginary” thought experiments assuming physical impossibilities that are yet logically consistent. His influence was especially strong in Italy in the fifteenth century, where his works were studied widely and commented on many times.