Emphasis can mean almost anything, bold, italics, a slowly articulated phrase, a close-up shot, a pink-haired anime character, etc. Here I will focus on textual emphasis, but these theories may be extended into many other domains, mutatis mutandis.
Each theory of emphasis defines its significance and provides a guide for which parts of a piece of text to emphasise. However, like anything in an arbitrary langauge of symbols, these are never theories in the objective sense, and might as well be called stylistic practices or modes of reading. I call them theories only because it better captures the univocity of communication between the author and the reader. As readers, we make inferences about what theory the writer holds, and accept the same theory for the purposes of understanding the text. However, each theory has its own side effects. A wrong choice of theory could hinder understanding or make the writer seem obnoxious.
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