Grammar as a phenomenon – a subsystem of language as a linguistic discipline

5. Parts of speech. Principles of classification
The words of language, depending on various formal and semantic features, are divided into grammatically relevant sets or classes. The traditional grammatical classes of words are called "parts of speech". These sets are specified in language, not in speech. It should be noted that the term "part of speech" is purely traditional and conventional, it can't be taken as in any way defining or explanatory.
Parts of speech are grammatical classes of words distinguished on the basis of three criteria: semantic, morphological and syntactic, i.e. meaning, form and function.
Principles/criteria for grouping words into classes
Meaning (Semantic criterion)
Each part of speech is characterized by the general meaning which is an abstraction from the lexical meanings of constituent words. (The general meaning of nouns is substance, the general meaning of verbs is process, etc.) This general meaning is understood as the categorial meaning of a class of words, or the part of-speech meaning.
Semantic properties of a part of speech find their expression in the grammatical properties. To sleep, a sleep, sleepy, asleep refer to the same phenomenon of objective reality, but they belong to different parts of speech, as their grammatical properties are different.
Form(morphological criterion/properties)
The formal criterion concerns the inflexional(form-building features- whiteness, development) and derivational features(word-building forms) of words belonging to a given class, i.e. the grammatical categories (the paradigms) and derivational (stem-building, lexico-grammatical) morphemes. ( Inflexional- categorial forms - > ability to have gram.categories and forms proper to a given cl.of sp. – flight -flights)
The classification criteria
Meaning Form
BUT!
There is no direct correspondence between gram. meaning and gram. form
This criterion is not always reliable as many words are invariable and many words contain no derivational affixes. Besides, the same derivational affixes may be used to build different parts of speech:
ly can end an adjective( a daily paper), an adverb(comes daily), a noun(a daily)
-tion can end a noun and a verb: to position.
Because of the limitation of meaning and form as criteria we mainly rely on a word's function as a criterion of its class.