I'll add my 2cts in the thread (though it seems to me to carry the idea that language -and/or communication- is central to design, not necessarily my conviction).
I would suggest that studies in rhetoric emphasize the means that allow a speach to convey a set of affects, rather than meaning (which is rather the purpose of semiology: the meaningfullness of a text). Rhetorics, since the greeks, has always been concerned with the distorsion between the discourse, and the embodiement of the response to it given by the audience (The problem of affects is a theoretically fascinating one, and has produced a lot of literature in its own time, it seems neglected nowadays). This is why the discipline has had a very mixed status, as a discipline that could, when well mastered, induce immoral acts and create illusory thruths.
It is quite interesting, from a semiotical perspective, to notice that the various tropes (metaphor, metonymy, allegoria etc) were called "figures" in some languages (I don't know if it was the case also in English), and the visual connotation has to be taken quite strongly : it induces the notion that there is a form of understanding that is morphological, rather than dialectical (which we all know and experience).
The dialectic link with illusion (which -to make it bold- is also related to the illusion created by the painter, which should not be a citizen of Plato's Republic) is, from a historical perspective, at the root of design. The ambiguous status of ornament and decorative arts since Renaissance and the various forms of criticism have constituted the discourse -in the sense of Foucault- of what is called design -eventually ending up in today's terminology of design is/as a process, rather then design is/as a value system that defines a subgroup of artifacts.
It is theoretically possible to define and analyze various rethorical forms in design : in the objects themselves, in the process itself (the representations -renderings etc- that are used, the langage of the presentations themselves, of scholars, critics etc). I am not sure that this would take us beyond the level of tautologies (I might of course be wrong) unless it takes into account 1/the variation in the meta-discourses that determine the territory of design (e.g.technology, utilitarianism) 2/the practices of users, with the drifts, shifts, and twists that makes the social meaning of each and every mass-produced artifact or service change constantly. Unless design redefines itself
Enough for today!
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