“Bhikkhus, suppose a man would throw a yoke with a single hole into the great ocean, and there was a blind turtle which would come to the surface once every hundred years. What do you think, bhikkhus, would that blind turtle, coming to the surface once every hundred years, insert its neck into that yoke with a single hole?” sn.v.456
“If it would ever do so, venerable sir, it would be only after a very long time.”
“Sooner, I say, would that blind turtle, coming to the surface once every hundred years, insert its neck into that yoke with a single hole than the fool who has gone once to the nether world would regain the human state. For what reason? Because here, bhikkhus, there is no conduct guided by the Dhamma, no righteous conduct, no wholesome activity, no meritorious activity. Here there prevails mutual devouring, the devouring of the weak. For what reason? Because, bhikkhus, they have not seen the Four Noble Truths. What four? The noble truth of suffering … the noble truth of the way leading to the cessation of suffering.
“Therefore, bhikkhus, an exertion should be made to understand: ‘This is suffering.’… An exertion should be made to understand: ‘This is the way leading to the cessation of suffering.’”