It's the old "those who know don't tell, and those who tell don't know" problem. Ultimate reality can't be captured in language, because language is dualistic: it discriminates... it separates "this" from "that". Actually, even that isn't quite right... the discrimination occurs prior to conceptualization, it's part of the basis of forming concepts to distinguish one thing from another thing. But reality isn't a collection of "things": "thing-ness" is something which arises in discrimination, which is a mental activity. Look at a tree... where is the boundary between the tree and it's environment? Normally we think of a tree as a "unit of being", but this is a very naive ontology. The tree is connected to the ground with a complex system of roots, and it exchanges gases with the atmosphere, and bugs crawl in and out, and liquids flow in and out, and the topology of the bark and branches is infinitely complex, and so forth. So where is this tree, exactly? What is it exactly? It's impossible to form an absolute boundary between the tree and not-the-tree. But with thought, with discrimination, we mentally separate the tree out and perform cognitive / logical operations on the SYMBOL for this infinitely complex / interconnected phenomenon. When we try to "pin down" the exact BEING of the tree, we can't do it. The mind just spins around in circles trying to grasp the essence of tree-ness. And that's just a simple example... reality is full of that sort of thing. In short, we can't really "parse" (chop up into pieces) reality without destroying it's wholeness. But we want to -- we really crave having a strong handle on our concepts about reality. We form concepts and cling to them for dear life. The mystics are trying to disrupt this clinging: "not this, not that" is about trying to pry our fingers off of the handles we've attached to it... to relax the clinging so that we can learn to just SEE what is right here, all the time -- absolute reality -- just as it is, unconditional, always the ground of being -- but entirely ungraspable. All we can say is "this is it". We can never answer "WHAT is it?" without producing paradox.