This question had the following example:
and the OP was asking about the meaning of は. I'd like to expand on this.
Firstly, I'm assuming that 上下は is part of 人間に上下はないと考える rather than being the topic for the entire sentence. It would seem strange to start with 人間に if the latter were the case.
That being so, my main question is, is it generally allowed to use は within an indirect quote? It feels weird to me. Probably because I know that (with exceptions) it isn't allowed in a relative clause. So, in the example sentence is the は allowed only because of the usual rule that ない likes to take は or is it just perfectly natural to have は in indirect quotes irrespective?
Finally, and perhaps this should be a separate question, I know that は is allowed in relative clauses for the purpose of contrast. Is it also allowed when making a negation such as 上下はない? I guess that is a form of contrast, but it's not clear to me.