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According to Egyptian myth, what part of Ma'at is weighed against a human heart to determine if a person is good or bad?
Feather.
A human heart is weighed on a scale against the feather of Ma'at, the goddess of truth and order. If a person is good, then the heart should be lighter than the feather and the soul is permitted to proceed to a blessed afterlife. If a person was bad, the heart would be heavier than the feather and the heart would be devoured by the monster Ammut.