The best sources for late ninth – and early tenth-century discussion of the problems of the Categories, Universals and essence are the sets of glosses to three works: Martianus Capella's De nuptiis, Boethius's Opuscula Sacra and, by far the most important, the Categoriae Decem. It is no accident that these glosses, especially those to the Categoriae Decem, also provide some of the fullest testimony to Eriugena's influence on his immediate successors. Each of these sets of glosses poses problems of authorship, which cannot even be formulated correctly, unless careful attention is paid to the nature of glosses, as opposed to literary texts.

Early medieval glosses are evidence for the methods and concerns of teaching and learning, not their conscious finished products. Sets of glosses may, in some cases, have been copied into manuscripts by pupils at their teacher's dictation; or they may all have been copied from one manuscript to another by the teachers or their scribes. At all events, each glossed manuscript of a school-text is the record of the interests of a particular centre at a given time. Different copies of the ‘same’ set of glosses vary, not simply in their selection of glosses, but in the very level of interpretation they lend to their text.