**If** a moral rule is grounded in shared human cognition, reflected in cross-cultural judgments, and justified by reasons any culture can accept, **then** it’s true for everyone; those conditions hold for “intentionally killing a non-threatening person is wrong,” **therefore,** global moral relativism is false.
This is an epistemological problem.
We have instinctive methods for trying to define principles that we use to go from a range of evidence and background knowledge to ideas that might be an explanation for them.
That’s our means to advance knowledge. But perhaps these are the wrong means, leading us away from understanding and knowledge, which means we’re stuck.