This is a sophisticated but important idea that will help you understand how ideas and meanings are being shaped, created or reinforced in a text. It is 'a theory of meaning' and an idea that can be applied to all texts; it is especially useful when analysing poetry where meaning has been 'compressed' into a very few words.
In the mid-20th century, two major European academic thinkers, Claude Levi Strauss and Roland Barthes, had the important insight that the way we understand certain words depends not so much on any meaning they themselves directly contain, but much more by our understanding of the difference between the word and its 'opposite' or, as they called it 'binary opposite'. They realised that words merely act as symbols for society's ideas and that the meaning of words, therefore, was a relationship rather than a fixed thing: a relationship between opposing ideas.
For example, our understanding of the word 'coward' surely depends on the difference between that word and its opposing idea, that of a 'hero' (and to complicate matters further, a moment's thought should alert you to the fact that interpreting words such as 'hero' and 'coward' is itself much more to do with what our society or culture attributes to such words than any meaning the words themselves might actually contain).
Other oppositions that should help you understand the idea are the youth/age binary, the masculinity/femininity, the good/evil binary, and so on. Barthes and Levi-Strauss noticed another important feature of these 'binary opposites': that one side of the binary pair is always seen by a particular society or culture as more valued over the other.
When studying any kind of literature, it is worth looking for the ways in which layers of meaning are being created, shaped or reinforced by this sense of 'binary opposition'. In Simon Armitage's poetry, for instance, you might notice the binary opposition he creates between the ideas we associate or attach to 'sincerity', 'genuineness' and 'truth' because of our culture's utter dislike of their binary opposites, 'insincerity' and 'lies'.
Recognising such binaries can open up the ideas the writer is trying to express. Look out for these oppositions as they can allow a deep understanding of what is happening in the text as well as alerting you to the 'big picture' - what it is all about.
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