Look at anything near you — a cup, a window, your hand. There seem to be two things: you, and what you see. Advaita, an Indian philosophy over a thousand years old, makes a startling claim: that division is not ultimately real.
Advaita means “not two.” Not “one” exactly — because “one” can still be counted — but simply the absence of a second. There is awareness, and everything appears within it, made of it, like waves made of ocean.
Tat tvam asi — That thou art.
This isn't a belief to accept. It's an invitation to look. Notice: thoughts come and go. Feelings come and go. Even the body changes completely over years. What has never come or gone? That which notices.
The teaching calls it Atman, the Self. And it says the Self is the same as Brahman, the ground of everything. Your deepest nature and the nature of the universe are not two.