Mu can mean 'nothing'. The absence of anything.
“I see nothing.” - Alice
“My you have good eyes.” - Cheshire Cat
"Do you hear silence?"
But even nothing must have great value.
One of mankind's greatest inventions was the wheel, yet the wheel itself, its spokes, and even its hub isn't of most value, but the center hole that drives the wheel.
There is no hollow in the vase without making the vase, no center hole without making the spokes and hub that define it.
It is to remind us that there is always a passive and empty component that coexists with an active, coercing and shaping component. The two provide the means towards action and purpose.
Nurture and acting.
Action is often coercion and coercion often manifests as aggression. If we can separate aggression from action we are not left with emptiness but inactive action. Wu wei.
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It may be used grammatically as a prefix. Not good. (Bad). Not day (Night).
But in Buddhism, Taoism, and Eastern religions, it can stand alone. Simply Not; an answer to a koan; an answer to unanswerable questions. Mu.
Unask the question. It indicates the question cannot be answered; that the question itself is at fault.
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I can't help but wonder if, in the center, the genesis point of all creation, there exists a Great Nothing. I think to attribute this as God – a Creator – is 'asking' the wrong question. It's looking at the situation from the wrong angle. For Creation itself, and even the act of creating itself, are controlling and coercive actions. Yang energy.
This central genesis point must be Yin energy. Non-coercive. Nurturing. Loving. Lacking aggression.
From a Theistic perspective, we really cannot have a God that is both omnipotent and omnibenevolent. We can, however, have a God that suffers from necessary, involuntary, and irreversible kenosis. Self-emptying.
... that is, of course, if we are forced to address this issue within a theistic context...
...but, like the banzai tree, what is the nature of this "divine" center; this divine Great Nothing?
What is the inactive action of this central nothingness?
What is God's wu wei?