My point is that accepting that something is a principal or eternal truth is subjective. We can’t see or hold objective truth in our hand. We can accept it or not. How do you prove what you call objective truth objectively? I’m not sure it can be done.
I am not arguing that point. I wish the new CAF told what a poster’s religion is, because an appropriate answer for Catholics is not the same as it would be for a protestant, and neither would be appropriate for an atheist, and nothing would be adequate for a radical skeptic.
For a believing Catholic, one would start a discussion from Church teachings in order to examine mores and values that flow from them; the teachings being the “principles” that are the starting point. The Church would argue that its teachings have their roots in revelation and right reason, but further that the Church has teaching authority granted by Jesus Himself.
For “biblical” protestants, the starting point would be the bible. But that can be shaky ground because Protestantism insists on personal interpretation of the bible. Nevertheless, agreement on at least some things can usually be arrived at; like the equivalence of “thou shalt not kill” with “thou shalt not murder”.
To an agnostic, it might be a reasonable starting point to talk of that which appears most likely to lead to an orderly society, or widespread prosperity or human psychology. And a common starting point might be found with an atheist on the same basis.
To a true skeptic, nothing will do because at least some of them deny that reality can be known at all, on any level. Some even deny the reality of their own existence or thoughts as thoughts.
Without knowing what another accepts or does not, it is really not possible to have a fruitful discussion. But I will add this much. If you believe all reality is as subjectively experienced and that there are no objectively true principles, then I very much doubt anything I say will cause you to believe anything at all.
The downside to such a belief, if widely held, is that ultimately only power really matters because in ordering society, nobody can really be counted on to respect the same things, and only the fiat of a ruler is relevant in determining how a society operates. That was Nietszche’s point in saying (if) “God is dead”, then the only resort in determining human conduct is to the power of the 'superman"; i.e. the person with the strength to impose his will or “values”.
It is exactly a foundational position of the Catholic Church that it really does teach objective truth. The application of those teachings is what we base our decisions upon in any given circumstance in which we reasonably ask ourselves what the moral choice is. Or at least we should. Catholicism also recognizes that we’re all sinners who do not do the objectively right thing a good part of the time.