So I have heard. At one time the Buddha was staying near Sāvatthī in Jeta’s Grove, Anāthapiṇḍika’s monastery. There the Buddha addressed the mendicants, “Mendicants!”

“Venerable sir,” they replied. The Buddha said this:

“Whatever dangers there are, all come from the foolish, not from the astute. Whatever perils there are, all come from the foolish, not from the astute. Whatever hazards there are, all come from the foolish, not from the astute. It’s like a fire that spreads from a hut made of reeds or grass, and burns down even a bungalow, plastered inside and out, draft-free, with doors fastened and windows shuttered. In the same way, whatever dangers there are, all come from the foolish, not from the astute. Whatever perils there are, all come from the foolish, not from the astute. Whatever hazards there are, all come from the foolish, not from the astute. So, the fool is dangerous, but the astute person is safe. The fool is perilous, but the astute person is not. The fool is hazardous, but the astute person is not. There’s no danger, peril, or hazard that comes from the astute. So you should train like this: ‘We shall be astute, we shall be inquirers.’”

When he said this, Venerable Ānanda said to the Buddha, “Sir, how is a mendicant qualified to be called ‘astute, an inquirer’?”

“Ānanda, it’s when a mendicant is skilled in the elements, in the sense fields, in dependent origination, and in the possible and the impossible. That’s how a mendicant is qualified to be called ‘astute, an inquirer’.”

“But sir, how is a mendicant qualified to be called ‘skilled in the elements’?”

“There are, Ānanda, these eighteen elements: the elements of the eye, sights, and eye consciousness; the ear, sounds, and ear consciousness; the nose, smells, and nose consciousness; the tongue, tastes, and tongue consciousness; the body, touches, and body consciousness; the mind, ideas, and mind consciousness. When a mendicant knows and sees these eighteen elements, they’re qualified to be called ‘skilled in the elements’.”