Every significant text of a grammatical exposition consists of a certain material, the vocabulary, and some structural properties, the style, of its author. The passive vocabulary is formed by the totality of all words of that language, s, the author writes in, the active vocabulary is formed by a certain set, s', of that totality, the selection of which is determined essentially by the sort of literature the text belongs to and depends only in a lower degree on the peculiarity of the author. Style, however, is characteristic of the author at a certain period of his personal development. The aim of the following investigation is to formulate mathematically some of the properties of structure constituting style, so that for a given text the application of a simple mathematical criterion allows its attribution to a particular author at a certain period of his mental development.