Not two — one.
Most people who have brushed against these ideas have met non-dualism: the recognition that the split between self and world, observer and observed, is not as real as it feels. Not two. The seer and the seen are one event.
Monism takes one more step.
Non-dualism dissolves the boundary. Monism asks what is left once the boundary is gone, and answers: one thing. Not a self facing a world with no wall between them — one organism, one field, one universe, briefly arranged into the shapes we call selves and worlds. “Not two” is a negation. “One” is the positive claim underneath it.
The usual objection arrives immediately: but I am obviously not you — I am sitting across the table. True, and beside the point. A liver cell and a neuron are wildly different and far apart, and they are one body. Distinct is not the same as separate. Two whirlpools in a river are genuinely two whirlpools and genuinely nothing but river. The distance you feel between yourself and everything else is real — it simply exists inside the one thing, not between two things.
Look for the edge of yourself and it keeps receding. Skin is a convention; you trade air and heat and water and light with the room without pause. There is no findable line where you stop and the world begins, because there is no such line to find.
Monism is not a mood or a belief to adopt. It is a description of what is already the case, arrived at from either end — the mystic’s and the microscope’s. They meet in the middle and describe the same one thing.