Lewis termed this abdication of high destiny, the 'abolition of man.'
Or, as Kierkegaard might put it, 'the abandonment of our true self.'
Lewis presents this philosophy in narrative form, in the story of Narnia, where beasts are gifted by Aslan with higher powers: the ability to talk, perceive the levels of reality and react accordingly. However, as the story develops, one notices that Talking Beasts can give up their privileged position and cease to view themselves as anything other than Dumb Beasts. They can view themselves “from below” and see their kinship with the Dumb Beasts. This freedom -- even to be less -- is part of their creation. 
Gilbert Meilaender comments:
However, in so doing they turn from the nature that is theirs. That is why, at the great scene of judgment at the end of Narnia, the Talking Beasts who look in the face of Aslan with hatred rather than love cease in that moment to be Talking Beasts...A freely chosen abolition of their nature occurs. 
When the powers of evil enter Narnia, some Talking Beasts begin to go bad. In Prince Caspian a bear attacks Lucy. After Trumpkin the Dwarf kills the bear, the children speculate whether at one time it might have been a talking bear.
“That’s the trouble of it,” said Trumpkin, “when most of the beasts have gone to the enemy and gone dumb, there are still some of the other kind left. You never know, and you daren’t wait to see.”
With horror, Lucy wonders if a similar process could take place in their world:
“Wouldn’t it be dreadful if some day in our own world at home, men started going wild inside, like the animals here, and still looked like men, so that you’d never know which were which?” 
This is the end of a demonized culture, where truth is exchanged for the lie and the proffer of true nature is rejected for the dumb, the counterfeit.
Here Lewis is incredibly profound and prophetic. It's like he looked into the soul of 21st century culture, and saw people -- good men and women, even -- choosing less than their calling, less than their high destiny. It's like he saw the terror of a world gradually going wild inside...
It is word both convicting and hopeful...
It calls us to choose our true selves in Christ, regardless of the gradual dumbing of our culture and the encroaching moral deafness of the world.
May we hear and be healed!
 Cf. Gilbert Meilaender, “The Primeval Moral Platitudes,” The Taste for the Other: The Social and Ethical Thought of C. S. Lewis (Grand Rapids: Eerdmans Publishing Company, 1998), 179.