A theory is exposed according to which the words' meanings must be referred to the structure of attentional moments. First of all, three "atomic categories" are obtained, which are the basis of all other categories, for both content and form. Then, through the operations of "metamorphization", "insertion" and "combination", twenysix elementary categories, corrisponding to the simplest meanings, are defined. This essay, that is the first chapter of a not yet published treatise, deals with the basic constitutive operations, from which it is possible to pass to the others, in order to propose a complete vocabulary.