Can mankind discern what is good and evil on its own? Certainly we could, but if a loving and wise parent wishes to give you advice on what is good and evil, why reinvent the wheel?
Said wheel is observed to be of flawed design.
Said loving and wise parent is observed using a wheel of different design.
Said wheel is determined to be invented not by said loving and wise parent, but by a monopolistic sibling.
If I saw those problems I might start to worry.
And if the advice of one you trust is determined to be incorrect?
Then you had best stop trusting them. But make sure you have the full explanation from them as to why their advice didn't seem to work right when you used it.
If no one agreed on exactly how fast gravity accelerates your feet to the ground it wouldn't mean that gravity doesn't in fact work.
Perhaps it would mean that "gravity" is not, in fact, a natural force, but some form of Intelligent Manipulation.
Care to expound?
This could be read either way - as people deliberately lying about how gravity works because it serves other agendas, or as gravity not actually working.
'How shall a man judge what to do in such times?' 'As he ever has judged,' said Aragorn. 'Good and ill have not changed since yesteryear; nor are they one thing among Elves and Dwarves, and another among Men. It is a man's part to discern them, as much in the Golden Wood as in his own house.'
And, of course, by whose standards is said parent loving and wise? I've seen people who think they are and are really vicious, petty, and heedless.
Well, that's interesting. I know that some are like that, and I know that I have made my best endeavour with my own two, to encourage critical thinking, and a rational view of the world. Yet still I see some parents carrying on what I consider to be a form of abuse, giving conflicting strictures, mostly because it suits their own agenda, it seems to me.
Freedom is the key for me and mine. We have the most outrageous conversations, and it is truly an honour to be regularly floored by a very pointy question from one of my kids.