From Terry Eagleton, Literary Theory: An Introduction, 2nd Ed. (Univ. of Minnesota, 1998), 82-83.

You can examine a poem as a 'structure' while still treating each of its items as more or less meaningful in itself. Perhaps the poem contains one image about the sun and another about the moon, and you are interested in how these two images fit together to form a structure. But you become a card-carrying structuralist only when you claim that the meaning of each image is wholly a matter of its relation to the other. The images do not have a 'substantial' meaning, only a 'relational' one. You do not need to go outside the poem, to what you know of suns and moons, to explain them; they explain and define each other.

Let me try to illustrate this by a simple example. Suppose we are analysing a story in which a boy leaves home after quarrelling with his father, sets out on a walk through the forest in the heat of the day and falls down a deep pit. The father comes out in search of his son, peers down the pit, but is unable to see him because of the darkness. At that moment the sun has risen to a point directly overhead, illuminates the pit's depths with its rays and allows the father to rescue his child. After a joyous reconciliation, they return home together.

This may not be a particularly gripping narrative, but it has the advantage of simplicity. Clearly it could be interpreted in all sorts of ways. A psycho analytical critic might detect definite hints of the Oedipus complex in it, and show how the child's fall into the pit is a punishment he unconsciously wishes upon himself for the rift with his father, perhaps a form of symbolic castration or a symbolic recourse to his mother's womb. A humanist critic might read it as a poignant dramatization of the difficulties implicit in human relationships. Another kind of critic might see it as an extended, rather pointless word-play on 'son/sun'. What a structuralist critic would do would be to schematize the story in diagrammatic form. The first unit of signification, 'boy quarrels with father', might be rewritten as 'low rebels against high'. The boy's walk through the forest is a movement along a horizontal axis, in contrast to the vertical axis 'low/high', and could be indexed as 'middle'. The fall into the pit, a place below ground, signifies 'low' again, and the zenith of the sun 'high'. By shining into the pit, the sun has in a sense stooped 'low', thus inverting the narrative's first signifying unit, where 'low' struck against 'high'. The reconciliation between father and son restores an equilibrium between 'low' and 'high', and the walk back home together, signifying 'middle', marks this achievement of a suitably intermediate state. Flushed with triumph, the structuralist rearranges his rulers and reaches for the next story.

What is notable about this kind of analysis is that, like Formalism, it brackets off the actual content of the story and concentrates entirely on the form. You could replace father and son, pit and sun, with entirely different elements — mother and daughter, bird and mole - and still have the same story. As long as the structure of relations between the units is preserved, it does not matter which items you select. This is not the case with the psychoanalytical or humanist readings of the tale, which depend on these items having a certain intrinsic significance, to understand which we have to resort to our knowledge of the world outside the text. Of course there is a sense in which the sun is high and pits are low anyway, and to that extent what is chosen as 'content' does matter; but if we took a narrative structure in which what was required was the symbolic role of 'mediator' between two items, the mediator could be anything from a grasshopper to a waterfall.

The relations between the various items of the story may be ones of parallelism, opposition, inversion, equivalence and so on; and as long as this structure of internal relations remains intact, the individual units are replaceable. Three other points may be noted about the method. First, it does not matter to structuralism that this story is hardly an example of great literature. The method is quite indifferent to the cultural value of its object: anything from War and Peace to the War Cry will do. The method is analytical, not evaluative. Second, structuralism is a calculated affront to common sense. It refuses the 'obvious' meaning of the story and seeks instead to isolate certain 'deep' structures within it, which are not apparent on the surface. It does not take the text at face value, but 'displaces' it into a quite different kind of object. Third, if the particular contents of the text are replaceable, there is a sense in which one can say that the 'content' of the narrative is its structure. This is equivalent to claiming that the narrative is in a way about itself: its 'subject' is its own internal relations, its own modes of sense-making.