  These are Buridan’s examples, used in the fourteenth century to show the invalidity of contraposition. Unfortunately, by Buridan’s time the principle of contraposition had been advocated by a number of authors. The doctrine is already present in several twelfth century  tracts,[11]  and it is endorsed in the thirteenth century by Peter of  Spain,[12]  whose work was republished for centuries, by William  Sherwood,[13] and by Roger Bacon.[14]  By the fourteenth century, problems associated with contraposition seem to be well-known, and authors generally cite the principle and note that it is not valid, but that it becomes valid with an additional assumption of existence of things falling under the subject term. For example, Paul of Venice in his eclectic and widely published Logica Parva from the end of the fourteenth century gives the traditional square with simple  conversion[15]  but rejects conversion by contraposition, essentially for Buridan’s reason. 