Andrew McLaren Lewis:
... What replaces it is an awareness that you are everything you see. If you see a mountain you think it is part of you. If you look at another person you think that you and he are one. He calls this One Taste ...
In my limited experience there are two stages of this. The first, of which I have a little bit of experience, is that the mountain is just part of your mind. The second, of which I only have an intuition or intellectual understanding, is that your mind is, in part, just the mountain.
This looks like a play on words, but it is not. If you think of the five aggregates, even if you separate out the body, the mind is still made up of sensation, perception, formations and cognition. I think the first stage of non-duality groks that perceptions just
arise in the mind, whereas the second stage groks that perceptions
are the mind - there is no witness. (For accuracy, this view of perceptions would include the mind sense.)
So, saying 'you and he are one' is not quite right. It's more that you are neither one nor separate (sounds like the Suttas!). It is recognsing that the idea of oneness or separateness is just a category mistake. Actually, all there is for us are the five aggregrates, and these aggregates make up the world including the mind and the 'self'. You and he are both just formations in the five aggregates, and you can label some of these 'you' and 'he' but that is a stressful process that is good to drop. (Yes there is likely be an external world too, but all we see of it are our sensations.)
As for the witness; I don't know for sure, but I looks like a provisional identification of self with formations and cognition, leaving sensations and perceptions 'out there'. Maybe it is useful as an intermediate step. But as Seth says, non-self experiences can occur at any stage of the path. So it may be helpful but I wouldn't identify it as the only way to progress.
I hope this is not a completely useless comment.