In Kanji in Context book, there's this one sentence:
人間に上下はないと考えるアメリカ人にとって、日本の目上と目下の関係は面倒だろう ?
I'm not sure why に and は and not が。 The second part of the sentence is understandable, but the first one makes no sense to me. I'm not sure what 上下 means either in this context. The reference book says "up and down" or "top and bottom," but I can't grasp the meaning.