Particle に and は


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.
  • Flaminius

    coclea mod
    日本語 / japāniski / יפנית
    は and が are often interchangeable. Here, -wa is preferred over -ga because the former doubles as the focus of negation.

    上下 is the distinction of class or status, an idea that all men are not created equal.