To speak of medieval semiotics is not to speak of a precisely defined discipline besides, and distinct from, other medieval arts and sciences; it is rather to speak of a complex field of more or less — mostly more — elaborate reflections on the concept of sign, its nature, function, and classification. In order to understand the enormous extent to which such theories grew during the Middle Ages some basic formal features of the scholastic organization of knowledge has to be kept in mind. First, scholastic learning is essentially a commentary tradition. Most of the writings either are explicit commentaries on what at a time were taken to be canonical texts (as e.g., the works of Aristotle, the Sentences of Peter Lombard, the Grammar of Priscian, or the Summulae Logicales of Peter of Spain or Buridan) or are at least composed with constant reference to the topics treated there. A second point, closely related to the first, is the common scholastic practice of putting great effort into the conceptual analysis of the basic terms and notions. Thus, wherever terms like ‘sign’ (signum) or ‘representation’ (repraesentatio) appeared in the texts commented on, scholastic authors felt obliged either to give an explicit account of these concepts or at least to be able to refer to a place where this has been done. In view of this, the fact that Aristotle in his On Interpretation had incidentally called the word a ‘sign’ (semeion, symbol) of the mental concept or that Augustine had termed the sacrament a ‘sacred sign’ (signum sacrum) became most important for the later development of semiotics. For in both cases the outcome was a large number of detailed explorations of the nature and divisions of sign.