"Writing, like any process of language, works by difference; but difference is not itself a concept, is not something that can be thought. A text may 'show' us something about the nature of meaning and signification which it is not able to formulate as a proposition."
"The advent of the concept of writing, then, is a challenge to the very idea of structure: for a structure always presumes a centre, a fixed principle, a hierarchy of meanings and a solid foundation, and it is just these notions which the endless differing and deferring of writing throws into question." (Eagleton 116)
So, language is basically volatile and uncertain. It cannot be bound to a specific structure, because it is always changing and the concept of language differs for everyone, much like Eagletons example of the meaning to the word 'cat' in the beginning of his essay. This is why we have metaphors and similes, because language itself can be manipulated.
Though I felt that Eagleton went on and on to explain post-structuralism, I think I get it. But, I don't like to say that with too much conviction, because I'm not completely sure. I have to admit my eyes began to hurt after the first few pages, but there were certain things that stuck out to me. These two quotes for example. I liked how Eagleton says that structure 'presumes' a center and foundation, because if language is so volatile and changeable then that center or that foundation can be shaken, or it can crumble. I think this is why we are never satisfied with one particular school of criticism. Though the basics seem to stay the same it still has to do with interpretation and what we discover inside the text. The foundation of language, so with writing, is always on shakey ground. This makes it rather exciting I think.