The identity-of-unit problem deals with various kinds of relationship between expression and content within a word. For example, synonymy, polysemy, homonymy.
Synonymy (a presumed singleness of content and variability of expression) serves as an instance to illustrate violation of the law of the sign which prescribes a direct correspondence between expression and content. Dictionaries of synonyms bring words together on the basis of relatedness of their meanings. In this process identity and differentiation, continuity and variability come to the fore as crucial parameters. It is customary to emphasize semantic distinctions between lexical units rather than their affinity. Dissimilarity of meaning and usage is important in learning to use synonyms properly.
The expression plane of semantically related words is synchronically quite incompatible, but nevertheless they tend to be centered on something in our inner mind’s eye – an isolated general idea, concept, or category. Therefore synonymy is closely connected with the notion of thematic groups – sets of words covering well defined fragments of extralinguistic reality.
The referential meanings of words are derived from reality and depend on how the conceptual space (a given referential area) is divided or covered by lexical items. For example, purchase and buy can be regarded as synonyms, but it would sound odd to say “I’ve just purchased a new dress”. So, purchase is associated with formal discourse, and buy is regarded as a more neutral option.
Synonyms which differ in respect of the varieties of discourse in which they appear can be defined as cognitive synonyms. In the Russian tradition we also come across the term “функционально-стилистические синонимы”.
Polysemy (singleness of form and multiplicity of content). The first way to establish polysemy rather that homonymy is to look for a central (or core) meaning. This is easier when we have examples of metaphor or transferred meanings. For instance, “sour” (having a sharp acid taste) acquires the meaning of “disagreeable”, as in: “They followed his gaze to find the sour joke”. Similarly the Russian adjective ‘’чистый’’ shows considerable variation: ‘’незагрязненный (о воздухе, воде), опрятный (об одежде), незапятнанный (об имени, репутации), прозрачный, неразбавленный (о жидкости), невинный (о человеке)’’, etc. In all these cases it seems possible to discover a central core of meaning which brings the lexical-semantic variants under a single general notion.
Homonymy (identity of expression and differentiation of content). Board 1 (a piece of wood), board 2 (a company council) and board 3 (meals). A similar instance of homonymy is presented by the Russian verb “топить”. Топить 1 (to heat) (печь, камин), топить 2 (to melt) (сало, масло, жир), топить 3 (to drown, to sink) (человека, судно, сети, грузила).
Polysemy and homonymy are not isolated but interacting aspects in the development of the word’s meaning. Variation within a word may bring to a stage when its semantic core is no longer elastic, it cannot be stretched any further, and as a result, a new word comes into being. This is the development of homonymy as the limit of polysemy.
Actually it is not always that the law of the sign happens to be violated (like in polysemy and homonymy): there are cases of smaller departures from the presumed agreement of expression and content when we speak of Variants (phonetic, morphological, or semantic) of one and the same word. A variety of the word’s meanings, however different or unlike each other, may have some semantic features in common to preserve and ensure the integrity of the word as a global whole. Let’s have a look at the phrasal verb “to weigh down” having the meanings of 1) to make somebody / something bend by being heavy, and 2) to make somebody feel anxious or depressed. Both of the variants are related to semantically to the verb “to weigh” by transfer of meaning and association. The two meanings of the word make us focus on both contrast and relatedness of senses.