On referring to Vaccarino's operational semantics, the author suggests a metalinguistic interpretation of mental categories, where univocal criteria are established and some key words are given to identify the structural differences of meanings, and to explain the corrisponding functions which make a structural nucleus to be a form or a content. The aim is to represent the elements for a classificatory pattern without any ambiguity of interpretation, and it should be reached by a "vertical" description of the categories' structures, following the concept of the increasing complexity going from the depht to the superficialness. The author concludes giving a list of the "elementary" categories, with the interpretation according to these criteria.