Symbolism. I've always thought that symbols are what makes us human. To be able to attach abstract meaning on the physical. To make an image into something greater, something that reaches for an understanding that is universal.
And symbolism is at the root of all literary devices, because it in fact is the root of language, of communication itself. Our words, we often forgot, are just vibrations in flying chaotically through the sky. Without symbolism, words are just sounds. It's the power of the symbol to give meaning where none exists.
And there is so much range in literature that a symbol can be just about anything. The format of the text itself can be a type of symbol as can a play inside a play as you said.
