Life is the most precious of treasures. Even one extra day of life is worth more than ten million ryo of gold. The Lotus Sutra surpasses all the sacred teachings of the Buddha's lifetime because of the "Life Span" chapter. The greatest prince in the land of Jambudvipa would be of of less consequence than a blade of grass if he  died in childhood. If he died young, even a person whose wisdom shone as brilliantly s the sun would be less than a living dog. So you must hasten to accumulate the treasure of faith and quickly conquer your illness.

from "On Prolonging One's Life Span"

Written to the lay nun Myojo in 1279