Thus have I heard. Once upon a time the Buddha was residing at Nadika, in the Brick Monastery, where he addressed the recluses, "Recluses, mindfulness of death when developed and made much of is of great fruit, of great benefit.
"It leads to the deathless [nirvana], has the deathless as its final end. How is mindfulness of death developed and made much of?
"There is the case where a practitioner reflects as day departs and night returns: 'Many are the causes of my death. Snake bite, scorpion sting, centipede pinch might be how my death would come about. And that would be an obstruction for me. I might fall, poor digestion, bile, phlegm [the humors of the body might become unbalanced]... piercing winds might be how my death would come about. And that would be an obstruction for me.'
"Then the practitioner should investigate: 'Are there any harmful, unskillful qualities still unabandoned in me that would be an obstruction if I were to die tonight?' If so, one should make an effort, arouse diligence [the opposite of negligence], endeavor, give undivided mindfulness and clear comprehension for the abandoning of those qualities.
"Just as when a person whose head is on fire would put forth effort and arouse diligence...to put out the fire, in the same way one should endeavor and abandon those qualities.
"But if by reflecting one realizes that there are no such states still left unabandoned, on account of that one should dwell in joy and rapture [abide in jhana], training day and night in skillful qualities [states or traits].
"There is also the case of one who reflects as night departs and day returns: 'Many are the causes of death....And that would be an obstruction for me.' Then one should investigate: 'Are there any harmful, unskillful qualities still unabandoned in me that would be an obstruction for me were I to die this day?' If so, one should make an effort...for the abandoning of those qualities.
"Just as when a person whose head is on fire puts forth effort and arouse diligence...to put out the fire, in the same way one should endeavor and abandon those qualities.
"Disciples, this is how mindfulness of death is developed and made much of so that it is of great fruit, of great benefit, leads to the deathless, and has the deathless as its final end."
That is what the Buddha said. And the recluses delighted in his words.