Death is certainly eventual.
Dying is not certainly gradual [as it can be sudden].
When you are only mindful of life,
you might not be mindful of death.
When you are also mindful of death,
you will be more mindful of life.
— Stonepeace | Get Books
In the Maranassati (Mindfulness of Death) Sutta, the Buddha reminded the monks that they should reflect at dusk, of how there are many possible abrupt causes of death, such as by being bitten by a snake or stung by a scorpion… which thus obstructs spiritual practice. Or one might die from a fall, food-poisoning or other ailments. One should therefore investigate if there are any defilements in the mind, that will impede one if one dies in the night, so that one can exert additional aspiration, mindfulness and effort in abandoning them, with urgency as if one’s head is on fire. If there are no defilements in the mind, one should dwell blissfully, and further one’s skilful qualities instead. Likewise, one should reflect so at dawn, on the possibility of death in the day, and practise accordingly. This is how mindfulness of death when developed is of great benefit, even leading to the deathless (Nirvana; liberation).
In another discourse with the same name, the Buddha remarked similarly to the monks that mindfulness of death is of great benefit, to which a monk says that he had already developed mindfulness of death. When asked how he did so, he replied that he reflects that if he might live for but a day and night, to practise what the Buddha teaches, he would have accomplished much. A second monk says he reflects in the same way, but in terms of a day. A third monk says that he reflects in the same way, but in terms of the interval it takes to eat a meal. A fourth monk says he reflects in the same way, but in terms of the interval that it takes to swallow after chewing four morsels of food. A fifth monk says he reflects in the same way, but in terms of the interval it takes to exhale after inhaling, or to inhale after exhaling.
The Buddha then remarked that whoever develops mindfulness of death, as according to the first four monks, are said to be dwelling heedlessly, as mindfulness of death is developed too slowly to end the effluents (mental fermentations of sensuality, views, becoming and ignorance, that ‘flow out’ from the mind to sustain the ‘flood’ of rebirth). Whoever, however, develops mindfulness of death with reflection that one might live for but for the interval it takes to chew one morsel of food, or as according to the fifth monk, for the interval it takes to exhale after inhaling, or the inhale after exhaling, to practise what the Buddha teaches, one would have accomplished much, by dwelling heedfully, as mindfulness of death is developed acutely to end the effluents.
In Section 38 of ‘The Sutra in 42 Sections’, a similar dialogue was recorded. ‘The Buddha asked a Shramana [monastic], “How long is the human life span?” He replied, “A few days.” The Buddha said, “You have not yet understood the way.” He asked another Shramana, “How long is the human life span?” The reply was, “The space of a meal.” The Buddha said, “You have not yet understood the way.” He asked another Shramana, “How long is the human life span?” He replied, “The length of a single breath.” The Buddha said, “Excellent. You have understood the way.” This is the most immediate mindfulness of death, about the fragile and fleeting nature of life, and the importance of living moment to moment with Dharma practice as much as we can. Although we do not always have a morsel of food in our mouths, we are always breathing, and with each breath, life shortens; the next breath might not come. As in the first Sutta above, death can be sudden too, with or without illness.
This day has already passed, and life has also reduced accordingly.
Like fish with lesser water, thus, what joy is there?
You should be diligent, like saving your head from burning.
Only be mindful of impermanence, and be careful not to be unrestrained.
– Samantabhadra Bodhisattva’s Verse For Admonition Of The Assembly