 The first two separate chapters of the Syncategoreumata are devoted to the words est and non, which are said to be implied in all other syncategorematic words. Peter’s account of the first word focuses on the notion of compositio (composition), which is explained in great detail, by looking into the signification of nouns and verbs (signifying a composition of a quality with a substance, and that of an act with a substance respectively). Considerable attention is given to the composition featuring in the verb ‘is’, in the form of the question of whether the composition involved can be counted among beings or not, considering the fact that it can be used to express different kinds of states of affairs. The chapter on negation introduces the important distinction between an act as conceived of or in the manner of a concept (ut concepta sive per modum conceptus) and as carried out (ut exercita) (see Nuchelmans 1988). Among the former type we find the noun ‘negation’ and the verb ‘to deny’, whereas the latter is what is meant by the negative particle ‘not’. The remainder of the chapter deals with the function of the negation, which is to remove the composition found in whatever it covers, and discusses some well-known sophisma-sentences which turn on the specific function of negation.