help paraphrase?

Question

the most plausible conception of ethics is not only compatible but also requires a certain amount of internal as well as external compromise

Answer 1

How about, "Understanding ethics requires compromise between yourself and society."

The only thing I'm not sure about is when you say "not only compatible".. compatible with what?

Answer 2

"Internal and external compromise" (C) is necessary for "the best ethical theory" (E).

(compatibility is derivative)

E→C. "If E, then C."

Anything further requires context.

To oversimplify: If ethics, then compromise
and: If no compromise, then no ethics.