Nothing is, though it seems to be

"Nirvana is a dimension not of this world nor of another world; neither coming nor going nor staying put."—Buddha Udana 8
"What we cannot speak about we must pass over in silence. … Facts are a thought."—Ludwig Wittgenstein, Tractacus

The Buddha taught that awakening was a kind of consciousness that doesn't experience itself as moving or stationary, a state wherein the mind doesn't land on anything, yet doesn't experience itself as unstable or moving. What does this mean? It's an awareness in which we're not committed to our habitual perceptions of life, in that we're not believing the normal idea of ourselves being a stable thing amongst a world of other stable things: there's "Me" and everything else that's "Not me." 

Now, this may sound abstract, but it boils down to a single and quite common observation: all things that appear or arise in the world must eventually pass, as something that is born or created is a matter of cells and atoms coming together, which constantly change or disperse. Things are impermanent, in that they are created and eventually disappear. Moreover, absolutely nothing maintains a lasting condition, for in the journey from arising to passing there is no real resting state; if we examine anything closely, even that which appears static motionless, perhaps a building or mountain, we'd see change. And most things reveal their impermanence with only cursory observation. What we see as "things" or "entities" are actually conglomerations of elements in change: that's what you, I, and everyone and everything else is.

Now, once we acknowledge that things that arise and pass, the processional nature of world makes the idea of "existence"—of anything truly "being" or "existing"—quite problematic. In other words: a person or that that is changing cannot be pinned down as a solid and stable entity, something with an identity.

Let's explore this: as the philosopher Wittgenstein observed in his Tractatus, the nature of language is committed to an idea of durability, that things can achieve a what could be called a state of being. Language is founded on the verb "to be" in which things achieve a condition of "is." These are words that denote stability, a lasting state. If I ask, "How are you?" and you answer "fine" you're presenting the idea that you've achieved, emotionally and ontologically, a state that is durable and static, and that your existence is "fine" or "suitable." Naturally, on a conventional level this may be a reasonable conversation, but its simultaneously creating a false belief: the illusion that your condition is static and steady, which it most certainly not the case. Any internal observation of your mind will reveal shifting and flowing thoughts, perceptions, feelings; an external observation of your body will reveal change as well. You are not one thing  from one moment to the next, though you've stated as much by saying you're "fine". You are not anything, in fact, over time.

So language, and the conceptual thought that grows from it, is based on that which is not real, for things don't really achieve a durable condition. It's the nature of reality to slip around what language can express. In reality, things neither exist nor not-exist, they are entities that are coming together and falling apart. So language and ideas, in terms of capturing real experience, is a futile effort.

Rather, the constant transience of all things offers a doorway to another way of perceiving life, in which identity is not a question of being, such as "I am." When we see things this way, awareness has nothing outside of itself on which it can land; and so the only resting place for consciousness is the quality of consciousness itself. Its less abstract than it sounds: because we're locked into the dualism of permanence that language presents, we're cut off from the emerging and vanishing mystery of existence. Rather than resting on anything solid, we rest on the way the mind observes the world, developing a receptive consciousness that doesn't demand the world to confirm to our ideas. The training is to suspend that kind of thinking and open up, instead, into touching the real experience of life itself, which is flux and flow.