There is no objective standard - I just believe it.
Perhaps this “self-evidency” is “written on our hearts”? Perhaps. Is it?
So if someone else has a different standard that they "believe" strongly in, it's just as valid as yours, like, sacrificing babies to a Mayan moongod, for instance.
We all have it, it was placed there by our Creator. The fact that you hear that voice telling you what is right and what is wrong speaks volumes. You are almost quoting St. Paul verbatim. Look up Romans 2:14, 2:15.