Spiritual

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As one deals with the language of identity and attempts to move closer to the oneness of consciousness by not nourishing the ego, what language can be used instead of "me, myself, I, etc.?
How does non-duality relate to emptiness? And is non-duality its own study? We have several sutras/texts on emptiness. Has non-duality been written about as well? Can you refer me to them?
What is spiritual discipline, and why is it so important?