I don't see a conflict between 'objectivity' and your second definition for morality, i.e "standards of behaviour; principles of right and wrong." The objectivity part doesn't require that we take human assessment completely out of the picture, but rather it requires that we leave out opinion. The question requires that one show that there are moral standards that all humans should be following. That requires an assessment so we'll always be in the picture, even if nature is the foundation for morality.
Can you name any human moral rule which is not based upon opinion?
Very good point to factor in. Non-human animals wouldn't have come up with their own moral standard, yet they seem to follow some standards of behavior. I've observed some animals even disciplining their young.
Perhaps it is the opinion on the parent that discipline is required. Are we to factor that in to the invention of morals?
The problem with making nature or biology the foundation of objective morality is that then it justifies the psychopaths or the male lions that kill the other male lions and their offspring just so they can take over the pride. Both are following their nature or biology. At best, I think we can say that morality is part of nature, but that alone does not tell us which morals are good or bad.
Is it good or bad that males lions kill off the competition?
Or is it simply that the opportunity presents itself to do so?
How would our world be if every male lion was spared this?
Sometimes I think that we "Disneyfy" the real world by superimposing our fantasies of a perfect world onto something we do not regard as a perfect world.
Perhaps therein, humans invented morality in order to try to force a perfect world onto the real one they have been experiencing.
If so, this opens up the question;
Q: "What is it about humans which has the ability to comprehend a [supposed] "Perfect World", which is so obviously different from the real world?"
We search for answers...
What have our sciences done to answer this question?
Or is it a matter that our sciences are being used specifically like unto the male lions, suppressing the main herd while they go about sailing into a particular direction they have selected for themselves?
For the herd notes, [for example] that as grandiose as the latest space telescope is - hurtling and unfurling [fully shaded] toward it's destination some million miles out and, simply to peer into the secrets of the past to 'try and understand'...the heard understands that the money could be 'better spent' on creating a perfect world here in the heart of imperfection - so why is that not been done?
Why is the rest of the herd being experimented on and used for that one purpose?
Just so a few lions can have their names recorded for all time?
Is that moral?
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Does it matter, since morals are really human inventions and are not aligned with the actual reality?
And to the Theist who might believe such, I would add a question to that one.
Q: Since this is not the perfect world you imagine, since you are thinking of kingdoms of plenty where this kind of thing cannot take place, why do morals matter hereabouts in this world, when they seem to serve better in these other imagined next level worlds?
For me in the middle, [Agnostic] I am somewhat undecided. I see the potential for humans to actually build a perfect world for themselves - irrespective of the chaos - and see those in the sciences attempting to do that.[through none other than the devices of the Sciences]
Unfortunately - not everyone is in favor of the perfect world envisioned - of the fiction-like story scientists are opening the door to...and so those not in favor are factored out, through invention...just like how the male lions deal with the male off-spring...not with morals but simply through the natural rule of the game-play of this reality...the School of Hard Knocks.