The problem is that we must judge ourselves. People have made as many claims about what the authority above the law (if such exists) says as about what the law says.
You are correct in that humans are stuck on earth with nothing but other humans to justify and provide earthly judgement. But to rephrase Blackstone's presumption - there is an authority that most of the people SUBSRCIBE TO, outside of the law. In that context, making law is relatively easy.
My belief is that civilizations that believe and act as though they are the final authority in defining right from wrong are not durable.
There's no doubt our problems have multiplied as our population has become more diverse (the Civil War occured because a fundamental differece of opinion on a very basic issue was not resolved when the Constitution was written). So far we've managed. One can only hope for the future.