Simply put, literary theory is an approach to thinking about reality.
Jacques Derrida's pluralistic perspectives differ from the gestalt centric suppositions of Carl Jung, yet both are unified and in vehement opposition to Structuralism. I would like to point to a path less traveled. Leibnitz, for example, was a monist. Strip out the spiritual and now we have a discussion worthy of further thought, especially in response to postmodern and post-structuralism.
Where does Marxism fit into this family of thought? The (claims of) historical inevitability of Marxism makes it a powerful and relevant perspective but one perspective only! Will it ever dominate? Um no, well... no. If history teaches us anything, Frankfurt 'schools the fool' from the position of THE official opposition. From a position of leadership, they look at each other and say, "Now what?".
Have we correlated aspects of personal responsibility from Sartre's Existentialism with and to the Protestant work ethic? This is a breeding ground for the propagation of planksip®. And for all you Nietzschean horse lovers out there, Freidrich Nietzsche is, despite being a powerful thinker, not really from this world, Krypton I think, let's leave it at that.
The above three paragraphs were fun to write. A mental warm up to the broader brushstrokes of Literary Theory. Let's start with aesthetics...