Lewis' guide through premise one
"How'd you like it if someone did that to you?" "I was there first." "But you promised!" Educated or ignorant, child or adult; anyone could have said any of these statements. The person wouldn't be saying, "your behavior makes me unhappy." The appeal is to a standard the other person already knows about. The other person never says, "to heck with your standard." Instead, they offer an excuse for why they broke the standard. The standard they are discussing is what allows human beings to argue. Animals cannot argue. We'd simply fight like animals without a common standard.
The idea of Decent Human Behavior is obvious to everyone. If this were not true, we may still have to fight “enemies” but we’d not be able to blame them any more than for the color of their hair.
Difference in civilization’s moralities have never amounted to a total difference. Compare the moral teachings of the ancient Egyptians, Babylonians, Hindus, Chinese, Greeks, and Romans. They’re strikingly alike. And like our own. (See the appendix to The Abolition of Man by C.S. Lewis.)
Name a country where cowardice is admired. Where people are proud of betraying those kindest to them. We may disagree on who you can be selfish toward and what degrees are okay, but we always agree selfishness is not a virtue.
Those who don’t believe in moral absolutes quickly cry out, “It’s not fair,” when a promise made to them is broken. We all know the rules. There is a Right and there is a Wrong. We may sometimes be mistaken, but it’s not a matter of taste or opinion.
What’s more – none of us keep the Law of Human Nature. Simply put: we ourselves do not practice the behavior we expect from others. Yearly. Monthly. Daily. “But…” we say. “I was tired.” “I needed a little extra money that month.” “I wouldn’t have promised if I’d known I was going to be so busy.” “If you only knew how irritating _____ can be.”
I don’t keep the Law of Human Nature, but. We’ve got excuses miles long. Don’t ask “are they good excuses” just yet. The fact is we wouldn’t need excuses if we didn’t very deeply believe in the law of human nature. We cannot bear breaking this law without shifting responsibility. We only offer excuses for our own behavior or those we try to protect. (Like a mom trying to protect her children from consequences.) We never have excuses for why we obey the Law.
In summary: A) all humans everywhere have the curious idea we ought to behave a certain way. We cannot dispose of it. 
Humans do not behave accordingly. “These two facts,” says Lewis, “Are the foundations of all clear thinking about ourselves and the universe we live in.” (21.)