The existence of such lexico-grammatical classes has never been a debatable issue among the linguists, although there are different views on the principles of classifications. The representative of the English scientific grammar H. Sweet stressed the need to take into account the meaning, form and function of words, but he emphasized the priority of the form over the other criteria. The representative of structuralism Ch.Fries emphasized the priority of the function (syntactic criterion).
In modern linguistics parts of speech are also differentiated either by a number of criteria, or by a single criterion.
Traditional classification
The polydifferential ("traditional") classification of words is based on the three criteria: semantic, formal, and functional. The semantic criterion presupposes the evaluation of the generalized (categorial) meaning of the words of the given part of speech. The formal criterion provides for the exposition of all formal features (specific inflectional and derivational) of all the lexemic subsets of a particular part of speech. The functional criterion concerns the typical syntactic functions of a part of speech. Contractedly the set of these criteria is referred to as "meaning, form, function" (Blokh, 2000).
In accord with the traditional criteria of meaning, form, and function, words on the upper level of classification are divided into notional and functional.
Prof. Blokh calls the notional parts of speech the words of complete nominative value; in the utterance they fulfill self-dependent functions of naming and denoting things, phenomena, their substantial properties (Prof. Blokh). Opposed to the notional parts of speech are the functional words which are words of incomplete nominative value, but of absolutely essential relational (grammatical) value. In the utterance they serve as all sorts of mediators (Blokh, 2000).
To the basic functional parts of speech in English are usually referred the article, the preposition, the conjunction, the particle, the modal word, the interjection. Functional words are limited in number. On the lines of the traditional classification they are presented by the list, each of them requiring its own, individual description.
The syntactic (monodifferential) classification