And what exactly are these 'objective external truths'
for example, raping then murdering a child purely for fun is morally wrong.. there you go, an objective moral truth
and how do you know them to be 'objective external truths'? Explain your methodology.
like I've said at least a dozen times, by cultivating and examining your individual moral conscience.
you know how it's done too; you do it dozens of times a day.. just like I don't need to explain to you a methodology for looking at a chair I don't need to explain to you how to examine your conscience.