Us-ness is the way you define, understand, and conceive of your own body. Who am I, really? Why am I here? What does it mean to live as a self inside of a body? We can only begin to answer these questions in, and by, and through our relationship with the other.
I do not exist, and cannot know myself (my us-ness), as an isolated individual (hence us-ness and not me-ness).
I only know the boundary that contains my us-ness by the constructed boundaries I – and we – create around otherness.