Ashes, or dry earth that has just been quarried, would share one color with his robe, and from
beneath that robe he drew two keys; the one

was made of gold, the other was of silver;
first with the white, then with the yellow key,
he plied the gate so as to satisfy me.

"Whenever one of these keys fails, not turning appropriately in the lock," he said
to us, "this gate of entry does not open.

One is more precious, but the other needs
much art and skill before it will unlock—
that is the key that must undo the knot.

...

Enter; but I warn you—he

who would look back, returns—again—outside. "

- D. Alighieri