The Genesis of the hidden people
One day God Almighty came to see Adam and Eve. They greeted him well and showed him all over the house. They showed him their children, too, and God thought highly of them. He asked Eve whether she had any more children beside these she had shown him, and she replied, no.
But the truth of the matter was that Eve had not finished washing some of the children and had been ashamed to let God see these. And for this reason she had hidden them away.
God knew this, however, and he said to her: "That which has been hidden from me shall be hidden from men."
These children now became invisible to mankind, and they lived in rocks and hills, stones and hummocks. From these hidden-children the hidden people descend, while mankind is descended from the children Eve showed to God.
Men can never see the elves, unless they themselves wish it, for they can both see men and let themselves be seen by them.