The author of the Trust in MindDaodejing uses it straightaway as a designation for a One that is beyond objectification; it cannot be known, yet it is suggested in all things. Zhuangzi uses it similarly, but in deference to the emptiness of the metaphysical Dao he also transforms it into a term descriptive of an experience, the experience of oneness. "Dao throughs as one." The experience of oneness is Dao.
The experience of oneness requires no One. Oneness is not experience of the One. The One does not grant the experience of oneness. A person does not experience oneness because there is One. A person experiences oneness because it is possible for the human brain to do so. There are no answers on offer.
Why then does one seek the experience of oneness? Because it feels good. If we seek it because it is our "birthright" we have rendered reality purposive, not empty. If we seek it because we 'should', we have rendered reality moral, not empty. If we say we cannot experience it if we have rendered reality purposive and moral, we have rendered reality conditional, not empty.
Oneness is not a metaphysical statement; it is an experience.
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