 Around the time William of Ockham (c. 1287–1347) wrote his Compendium of Logic (Summa logicae), medieval logic began to change. More emphasis was placed on the theory of consequences than the theory of syllogisms. A theory of consequences was developed by Abelard in the course of his discussion of topical inferences and hypothetical syllogisms, and during the thirteenth century the basic idea was further developed in treatments of the topics, but in the fourteenth century works devoted solely to consequences began to appear (Green-Pedersen 1984). The most famous is John Buridan’s Treatise on Consequences (Tractatus de consequentiis), though earlier authors such as Walter Burley had also stressed consequences over syllogisms. Burley probably wrote his On the Purity of the Art of Logic (De puritate arte logicae) as a reply to Ockham’s famous Summa. For Ockham, however, syllogisms are still the most important formal inferences, and he devotes most of Book III of the Summa to them (see Normore 1999 for a recent study of Ockham’s logic). 