A fundamental tenet of postmodernist theory is that reality is unknowable or, rather, what one experiences is a series of words or ideas (often referred to as signs) that make reference to things in the world. What we know is the sign but not what it points to (i.e. the thing in the world).
This is an old problem in philosophy concerning the limits of knowledge: if what one knows is knowable only through one’s senses then how does one have knowledge of the thing itself (not just a set of properties one perceives)? In fact some philosophers deny that there is a thing itself as we never have any direct experience of it.
For instance, if I look at my desk I see certain attributes regarding its color and its proportion. If I change position, the color and proportion of the desk change as well due to perspective shifts. The dilemma is what does the desk look like. How can I know what the real desk looks like if all I experience is thousands upon thousands of different views?
This issue was brought to the forefront just as Empiricism was gaining a foothold. Indeed one can see a visual depiction in Cubist painting. Post-modernism, however, took the dilemma to heart and used it as a basis for generating art.
