Lao Tzu the Chines philosopher in his book Tao Te Ching rhigorously explored the duality of nature best summed up by the symbol of the Tao, the Yin and Yang characters. The nature of the Universe is expressed by opposites(in kind) that compliment and complete each other; Black vs white, light vs darkness, fullness vs emptiness, male vs female, hot vs cold, up vs down, in vs out, left vs right, hard vs soft, wet vs dry, the infinitely large vs the infinitely small, etc… In fact the existence of one requires the coexistence of the opposite quality.
If everyone was the same height(for example) then there would be no tall or short in regards to stature. If everyone was equivalently mentally adept then there would be no smart or dumb. Because we have Polarity we must have Duality. Where there is no Polarity(in kind)then there is no Duality.
It has been established by measurements of Space that the Universe is both Infinite and Eternal. Because of this there was no beginning and no creation of the Universe and we can only conclude that the invention of a nonphysical was merely one of convenience to invoke the concept of God to explain what clearly never occurred by an entity that never existed. For a creator to be the Universe has to be finite.
We can also now conclude that there is no Nonphysical Reality. Everything is Physical, mind, body and spirit. And because of this there is no Polarity between Physical and Nonphysical because the Nonphysical doesn’t exist. There is no Paranormal, no Ghosts, no Telepathy, no Voodo, and no Magic except as far as deceptions by fraudsters is concerned.
The only conclusion then is that Duality only exists as a description of physical reality, Nature. And that Nonduality is the truth that there is no Nonphysical Reality to oppose natural phenomena.