  The history of philosophy remembers William for his two theories of universals, material essence realism and indifference realism.  These theories emerge from the writings of Peter Abelard, where they are paired with Abelard's decisive arguments against them. Indeed, Abelard's critique was so powerful that when John of Salisbury wrote his famous catalogue of twelfth-century theories of universals, William is not even mentioned (Metalogicon II 17-20; Abelard LI Por.: 10-16, trans. Spade 1994: 29-37; see also  John of Salisbury,  Peter Abelard, and  King 2004)