Consciousness is never experienced in the plural, only in the singular. Not only has none of us ever experienced more than one consciousness, but there is also no trace of circumstantial evidence of this ever happening anywhere in the world. If I say that there cannot be more than one consciousness in the same mind, this seems a blunt tautology — we are quite unable to imagine the contrary...

"The Oneness of Mind", as translated in Quantum Questions: Mystical Writings of the World's Great Physicists (1984) edited by Ken Wilber