My current thinking is that, since normative principles (what ought to be) cannot be demonstrated to be universally true, the best we can do is demonstrate that they are consistent with our core instincts and motives. For example, being generous to others is not right based on some external principle, but because if I had sufficient emotionally maturity, security, and insight I would recognize that my deepest desires extend far beyond immediate gratification of my physical desires.

[0]: https://moralfoundations.org/ [1]: https://en.wikipedia.org/wiki/The_Righteous_Mind