Barthes starts his text by comparing some analyses of narrative to Buddhists ascetic practices: both are trying to find a universal model that would simplify all the complexity of the world in one, and the text in the other case to one universal scheme. As the word ascetic implies, such schemes are merely skeletons of the ideas they are trying to embody. We cannot really see the world in a bean, simply because we are not aware of everything at every moment. Scio me nihil scire - claiming the opposite would mean being unaware of one's own unawareness.
So far Barthes seems to advocate the "writerly" - complexity of the text and the idea that the process of reading is or should be the process of "writing". We construct the text as we read it, Barthes explains. Then he offers a topos, or a convention, of how we "write" a particular text as we read it. He offers us five codes for that. He explains the system of associations that help us understand what we read. There is only one problem with the theory of five codes - it seems to be fairly ascetic.
Barthes' five codes operate within boundaries of our logic, expectations and, arguably, Jungian "collective unconscious". For example, hermeneutic code directly depends on logic that forms our expectations - we do not ask questions that we do not find logical because they do not seem necessary or even probable to us. Similarly to that, symbolic code operates within the boundaries of the archetypal or "collective unconscious" - certain symbols have preset meanings according to which they are deciphered. Now, knowing that we only have five codes and that every code operates by the already existent rules, it seems that there is no way out of a bean. We have a nice little scheme that allows explosion of meanings only within its own limits. However, claiming absolute understanding of the principles of text equals claiming that we can see the world in a bean. Yet, it is what we often do. Adopting Barthes' rules themselves means limiting his text to a mere scheme because we are simply choosing denotative meaning of his text over every other.
It is also significant that, to prove his point about the five codes, Barthes uses Balzac's story. Balzac is also often called the "father of the Realist novel" and his prose is a textbook example of social criticism in literature. Bearing that in mind, we are not very likely to expect that Barthes would use Balzac's story to prove that the process of reading is actually the process of "writing" as Balzac's intent was to capture and portray social problems of his time as they were. However, the fact that Barthes still chooses Balzac suggests that literally everything is a process of "writing" and projecting our own views onto something else. Hence, text as a system of references is not something characteristic only of our time and in that sense post-modernism is not revolutionary.