The crux of Advaita Vedanta non-dualism explained in 13 minutes (from minute 43 - 56).

https://youtu.be/sD6MgSiRk9A?t=2586

This is the Vedantic method of looking within, or Neti Neti. It's one of many Vivekas, this one being the seer and the seen, negating what one is not and leaving the one unchanging aspect that one is.