Metaphor is the use of an expression with a meaning which is similar to the original meaning. In what follows, the base meaning is M1, the target meaning is M2. This is another process which acts on the synchronic and the diachronic axis. When speakers start applying it to an expression, they increase its polysemy. The expression then has more than one meaning on the synchronic axis. Further semantic change may then suppress M1, so that only the derived metaphorical M2 survives. Then the relation between M1 and M2 is a diachronic relation.

The concept of metaphor is well-known from lexical semantics, as when we speak of the key to the solution, where M1 is a certain physical object that opens a lock, thus providing access to something behind it, while M2 is anything which provides access to something. Metaphor is an integral component of the desemanticization operative in grammaticalization. Here are some examples:

• In the grammaticalization of be going to V, the base construction designates physical movement, as in be going to work. In be going to read, the move is metaphorical, it is just a developing process.
• The recent past in French is a periphrastic construction of the structure venir de V-INF, lit. ‘come from Ving’. Here again, physical motion to the deictic center is the basis for a transfer to a process which leads up to topic time.
• The Yucatec noun táan ‘middle’ grammaticalizes to a progressive auxiliary, via a metaphor which locates the aspectual perspective in the temporal “middle” of a situation.
• In an Old French combination like ne marcher ‘not march’, the inherited Latin negation ne is reinforced by the addition of the noun pas ‘step’ to produce ne marcher pas ‘not march a step’. Pas then becomes the basis of a metaphor where it means ‘a single phase in a process, a bit’, used, e.g., in ne manger pas ‘not eat a bit’. With further grammaticalization, this then becomes a discontinuous negator, so the example just means ‘not eat’.
• The relational noun side has the basic meaning ‘lateral part [of something]’. The preposition beside based on it has the basic meaning ‘in the lateral region [of something]’, as when John is standing beside Linda. When this preposition is grammaticalized, it acquires, by metaphor, the additional meaning ‘as compared with’, as in beside Leonardo, all other renaissance draftsmen are second-rate.

Lexical metaphors are rather diverse in terms of the semantic changes involved. Here a major distinction will be made between two ways of applying an expression with basic meaning M1 in a context where its meaning is M2:

1. The most specific semantic feature of M1 may be eliminated. M2 thus automatically becomes a hyperonym of M1. Beside the subset of entities fulfilling the condition dropped, this hyperonym then also designates entities that differ from the former with respect to this condition. A frequent case is dropping the features characterizing a physical object so M2 applies to anything found in the same abstract relation as M1. This happens in all of the examples adduced above.
2. M1 may have conventional implicatures. These are then taken as the characteristic semantic component of M2. This happens in the metaphor my knight is a lion in the battle, where the property of being strong and bold is not a semantic feature of the noun lion, but a cliché associated with its designatum. It also happens in the grammaticalization of on top discussed in the previous section.

Changes of type #a are characteristic of grammaticalization. Changes of type #b do occur in grammaticalization, but similarly as a phonological change may be contemporaneous with a process of grammaticalization without affecting its nature.

To the extent that metaphor can be described as meaning extension (type #a), a subtype of it which is characteristic of many grammaticalization paths should be noted. This is the change of a local to a temporal meaning and, more general, from a concrete to an abstract meaning. The former is typical of the evolution of many case relators. For instance, Engl. after X is, by its etymology, ‘behind X’, but now also designates a temporal relation. The same goes for at, which designates a local relation in at the house, but a temporal one in at noon. The shift from local to temporal also occurs in the evolution of aspectual formatives, e.g. in the case of Yucatec táan mentioned above. This development goes always from local to temporal¹ and has even given rise to the general hypothesis of localism (Fortis 2020, with additional references). The even more general change from a basic concrete to a metaphorical abstract meaning characterizes the development of French pas mentioned above, as well as the development of the prepositions on and under in such collocations as on condition and under the condition. Since these changes are essentially irreversible, it seems possible that a theory of semantic representation would not describe them as replacements of local by temporal or of concrete by abstract features, but instead by just dropping the conditions locality and concreteness, resp.

¹ Opposite changes, from temporal to local, if they occur, will be based on the analogy with polysemous spatio-temporal adpositions.

References

Fortis, Jean-Michel, 2020, ‘From localism to neolocalism’. Aussant, Émilie & Fortis, Jean-Michel (eds.), Historical journey in a linguistic archipelago: Descriptive concepts and case studies. Berlin: Language Science Press; 15–50.

Taverniers, Miriam 2018, ‘Grammatical metaphor and grammaticalization. The case of metaphors of modality’. Functions of Language 25: 164-204.