In this paper we study the selection of cognitive elements and criteria of the inflectional structure of the Russian and Arabic languages in the process of speech communication. Phonetic-physiological principle is the main parameter by which the elements and criteria of cognitive activity in the presented study are distinguished. On the basis of the above mentioned parameter, we select the investigated criteria and elements. The first criterion is semantic, reflects the accordance of the elements of thinking to sound combinations in the studied languages, and allows us to distinguish the second criterion – morphonological. The second criterion depends on the phonetic changes of these combinations occurring in the process of speech activity, and, coupled with the previous one serves as a reference for the selection of the third – syntactic criterion, which is revealed when considering the inflectional structure of the language. The set of the above elements and criteria forms a coherent cognitive chain of speech activity . The inflectional structure of the Russian and Arabic languages demonstrates that they can be distinguished by various elements and criteria of cognitive activity, starting with the minimum element – the sound, which by its nature is transmitted by means of phonetic waves and has different frequency vibrations. In the process of communication, a set of sounds is perceived as a matrix of elements with different vibration frequencies; these elements, being in a certain order, reflect the lexical meaning of the word perceived by ear.