Language is a cognitive and communicative activity. It is structured in the form of language systems, i.e. systems of individual ‘langues’. The language system is a semiotic system. As such, it is the result of the interplay of two essentially independent forces:

1. Formal constraints on structure: The constraints on a semiotic system and on the messages constructed with it are of a heterogenous nature. Laws of logic and information theory determine how signs may be selected and combined. Laws of physics determine the composition and transmission of messages. These are complemented by other laws of nature in the case of semiotic systems used by a particular species, e.g. Homo sapiens.
2. Functions of communication and cognition: The world surrounding us which we conceptualize is in many respects the same for every speech community; and the same goes for the tasks of communication in such a community. The functions of cognition and communication provide the total of content and its conveyance in the widest sense.

Thus, entities of grammar have a purely formal side determined by the constraints imposed on any semiotic system. At the same time, this formal side is not empty, but is laden with cognitive and communicative content. In more concrete terms: Grammatical categories, relations, constructions and operations are necessary for a semiotic system to operate, and they do have some purely formal properties. At the same time, those are categories like tense, relations like the indirect object relation, constructions like the causative construction and operations like nominalization; and none of these is purely formal, all of them have their semantic side. Putting it yet another way: in a semiotic system, everything concerning the sign as a whole is significative.

The double-sidedness of grammatical entities has many methodological consequences. Two are of immediate relevance for language description:

• Definitions of a grammatical category are mixed definitions (occasionally called 'hybrid'), combining semantic and structural criteria.
• Any analysis of grammatical categories aiming at understanding their nature has to take a double approach to them, a formal and a functional approach.

Semasiological grammar takes the formal approach, onomasiological grammar takes the functional approach.