The Feather of Maat

The Ancient Egyptians believed that when you died, you travelled to the Hall of the Dead.
There Anubis weighed your heart against the feather of Ma'at.
If your heart was lighter than the feather, you lived for ever.
If your heart was heavier than the feather then it was eaten by the demon Ammit, the Destroyer.

We still talk of "a heart as light as a feather" to mean care-free, and "heavy-hearted" to mean sad.

From http://gwydir.demon.co.uk/jo/egypt/anubis.htm

 

“Evil words and deeds made the heart heavy, and after death Anubis weighed your heart on a balance scale against the Feather of Maat. If your heart weighed more than the feather, the baboon-headed god, Thoth, tossed it to the monster Ammut to eat.

From The Book of the Dead, by Douglas Preston, Lincoln Child