This book examines the various philosophical influences contained in the ancient description of the noun. According to the traditional view, grammar adopted its philosophical categories in the second century B.C. and continued to make use of precisely the same concepts for over six hundred years, that is, until the time of Priscian (ca. 500). The standard view is questioned in this study, which investigates in detail the philosophy contained in Priscian’s Institutiones grammaticae. This investigation reveals a distinctly Platonic element in Priscian’s grammar, which has not been recognised in linguistic historiography. Thus, grammar manifestly interacted with philosophy in Late Antiquity. This discovery led to the reconsideration of the origin of all the philosophical categories of the noun. Since the authenticity of the Techne, which was attributed to Dionysius Thrax, is now regarded as uncertain, it is possible to speculate that the semantic categories are derived from Late Antiquity.