In the present paper we characterize in more detail some of the aspects of a question answering system using as its starting point the underlying structure of sentences (which with some approaches can be identified with the level of meaning or of logical form). First of all, the criteria are described that are used to identify the elementary units of underlying structure and the operations conjoining them into complex units (Sect. 1), then the main types of units and operations resulting from an empirical investigation on the basis of the criteria are registered (Sect. 2), and finally the rules of inference , accounting for the relevant aspects of the relationship between linguistic and cognitive structures are illustrated (Sec. 3).