If one compares proverbs and phraseological units in the semantic aspect, the difference seems to become even more obvious. Proverbs could be best compared with minute fables for, like the latter, they sum up the collective experience of the community. They moralise (Hell is paved with good intentions), give advice (Don't judge a tree by its bark), give warning (If you sing before breakfast, you will cry before night), admonish (Liars should have good memories), criticise (Everyone calls his own geese swans).
No phraseological unit ever does any of these things. They do not stand for whole statements as proverbs do but for a single concept. Their function in speech is purely nominative (i. e. they denote an object, an act, etc.). The function of proverbs in speech, though, is communicative (i. e. they impart certain information).
The question of whether or not proverbs should be regarded as a subtype of phraseological units and studied together with the phraseology of a language is a controversial one.
Professor A. V. Koonin includes proverbs in his classification of phraseological units and labels them communicative phraseological units (see Ch. 13). From his point of view, one of the main criteria of a phraseological unit is its stability. If the quotient of phraseological stability in a word-group is not below the minimum, it means that we are dealing with a phraseological unit. The structural type — that is, whether the unit is a combination of words or a sentence — is irrelevant.
The criterion of nomination and communication cannot be applied here either, says Professor A. V. Koonin, because there are a considerable number of verbal phraseological units which are word-groups (i. e. nominative units) when the verb is used in the Active Voice, and sentences (i. e. communicative units) when the verb is used in the Passive Voice. E. g. to cross (pass)