Everything that you experience in consciousness is non dual. That means it is not a second thing apart from you. In this vast universe, there is only one reality, existence isness. And if you trace the sense of existence, whit things and people, is is is, very soon you will see the isness is not out there. The isness is your isness. I am. All this is this isness, this existence, it itself appears as a multitude of beings, the self in all beings. It is existence, which is yourself. Who says, existence is myself? Nobody doubts that.
Inspired by the 15th chapter of the Ashtavakra Gita, Swami Sarvapriyananda gives a discourse on the nature of the Self.