 Peter of Spain has been established as the medieval author of a work that became widely known as Summule logicales magistri Petri Hispani (Collection of Logic Matters of Master Peter of Spain). The great number of manuscripts and printed editions is evidence of the enormous success this work met with throughout European universities well into the seventeenth century. An interpolated version of his Tractatus, by then known as the Summulae logicales, was used by John Buridan as a basic text to comment upon. But finding out the true identity of the author of this influential Tractatus has proved to be a difficult task. For a long time it was assumed that he was a Portuguese who became Pope in 1276, under the name of John XXI. There is also another, earlier tradition, according to which the author of the Tractatus was regarded as Spanish, and a member of the Dominican order. Yet another attribution, dating from the fifteenth century, was to a Petrus Ferrandi Hispanus, who died between 1254 and 1259; this attribution would be consistent with the idea that Peter’s works on logic originated from the first half of the thirteenth century. According to still another attribution, the Summule was compiled by a Black Friar no earlier than in the late thirteenth or early fourteenth century.