This paper accounts for the semantics of plurality, in particular, the accessibility relation for plural anaphoras. Two types of dis- course antecedents formations, inherited from the classical treatment [14], namely summation and abstraction, are studied in detail. Solutions for each phenomenon are provided respectively by introducing two new functions Sum and Abs, for obtaining the semantic interpretations. The technical background in this paper is based on a recently proposed dynamic semantic framework [9], which pertains a style of the traditional Montague Grammar and the principle of compositionality, without adding any new concept to classical logic.