The author defines the semantic category of diathesis as grammaticalized information about the hierarchy of arguments inscribed into the semantic structure of a predicate. She demonstrates that we can perceive an event from different perspectives depending on which argument is for the moment in the center of our interest. Thus, unlike aspect, mood or tense, diathesis is not an inflectional category of the constitutive predicate of a proposition, but a category of a pro-position as such, notwithstanding the fact that there are oppositions as active ~ passive, or possession ~ appertainance/belonging to which affect directly the surface form of the constitutive predicate. There is also something as a natural diathesis depending on the semantic role of the top argument - it is characteristic of propositions with argument referring to the agens at the top of the hierarchy. Understood this way diathetical hierarchy can serve as a criterion for a fun-ctional classification of propositions and the place of an argument in that hiararchy as a criterion for a functional classification of arguments.