The Buddha uses a simile of a man who can catch the arrows shot by well-trained archers before they touch the ground to illustrate that life-sustaining conditions wear away faster than that.
At Sāvatthi.
"Bhikkhus, suppose there were four archers, firm in their aim, well-trained, skilled, and accomplished in their craft, stationed in each of the four directions. Then a man might come along and say, 'I will catch the arrows that these four expert archers, firm in their aim, well-trained, skilled, and accomplished, shot in each of the four directions before they touch the ground and bring them back.'
What do you think, bhikkhus, would it be appropriate to say, 'This man is one possessed of the utmost speed?'
"Venerable sir, even if someone were to retrieve the arrow shot by one expert archer, firm in their aim, well-trained, skilled, and accomplished, before it touched the ground, it would be appropriate to say, 'This man is possessed of the utmost speed.' How much more so in the case of four such archers, firm in their aim, well-trained, skilled, and accomplished?"
"Bhikkhus, though the speed of that man is fast, the speed of the moon and the sun is faster than that. And faster still than the speed of the moon and the sun is the speed of the deities who run ahead of them. But faster still than all of these is the [moment-by-moment] wearing away (exhaustion, depletion [khīyati]) of the life-sustaining conditions (life forces, vital formations [āyusaṅkhārā]).
Therefore, bhikkhus, you should train yourselves thus: 'We will dwell diligently (doing one's work or duty well, with alertness, carefulness and care [appamatta]).' Thus, bhikkhus, you should train yourselves."
Related Teachings:
Life Is Short and Fleeting (AN 7.74) - Araka was a famous teacher long ago, when the life span was much greater than today. Nevertheless, he still taught impermanence; how much more is it relevant today!
Rare That One Obtains the Human State (SN 55.47) - The Buddha shares on the difficulty of regaining human existence for an undiscerning person who has fallen into a lower realm.
One devoted to diligence is incapable of decline (DhP 30, 31, 32)
