This is the second discourse from the Buddha to the five mendicants. Here, the Buddha shares suitable ways to reflect on the five aggregates.

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At one time the Buddha was staying near Varanasi, in the deer park at Isipatana. There the Buddha addressed the group of five mendicants:

“Mendicants!”

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

“Mendicants, form is not-self. For if form were self, it wouldn’t lead to affliction. And you could compel form: ‘May my form be like this! May it not be like that!’ But because form is not-self, it leads to affliction. And you can’t compel form: ‘May my form be like this! May it not be like that!’

Feeling is not-self …

Perception is not-self …

Choices are not-self …

Consciousness is not-self. For if consciousness were self, it wouldn’t lead to affliction. And you could compel consciousness: ‘May my consciousness be like this! May it not be like that!’ But because consciousness is not-self, it leads to affliction. And you can’t compel consciousness: ‘May my consciousness be like this! May it not be like that!’

What do you think, mendicants? Is form permanent or impermanent?”

“Impermanent, sir.”

“But if it’s impermanent, is it suffering or happiness?”

“Suffering, sir.”

“But if it’s impermanent, suffering, and perishable, is it fit to be regarded thus: ‘This is mine, I am this, this is my self’?”

“No, sir.”

“Is feeling permanent or impermanent?” …

“Is perception permanent or impermanent?” …

“Are choices permanent or impermanent?” …

“Is consciousness permanent or impermanent?”

“Impermanent, sir.”

“But if it’s impermanent, is it suffering or happiness?”

“Suffering, sir.”

“But if it’s impermanent, suffering, and perishable, is it fit to be regarded thus: ‘This is mine, I am this, this is my self’?”

“No, sir.”

“So you should truly see any kind of form at all—past, future, or present; internal or external; coarse or fine; inferior or superior; far or near: all form—with right understanding: ‘This is not mine, I am not this, this is not my self.’

Any kind of feeling at all …

Any kind of perception at all …

Any kind of choices at all …

You should truly see any kind of consciousness at all—past, future, or present; internal or external; coarse or fine; inferior or superior; far or near: all consciousness—with right understanding: ‘This is not mine, I am not this, this is not my self.’

Seeing this, a learned noble disciple grows disillusioned with form, feeling, perception, choices, and consciousness. Being disillusioned, desire fades away. When desire fades away they’re freed. When they’re freed, they know they’re freed.

They understand: ‘Rebirth is ended, the spiritual journey has been completed, what had to be done has been done, there is no return to any state of existence.’”

That is what the Buddha said. Satisfied, the group of five mendicants approved what the Buddha said. And while this discourse was being spoken, the minds of the group of five mendicants were freed from defilements by not grasping.

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At the conclusion of this discourse, all five mendicants become perfected ones. The three ways the Buddha shares to reflect on the five aggregates:
1. Impermanence,
2. Discontentment,
3. Not-self

are self-sufficient, and suitable ways to realize Nibbāna (enlightenment).

The characteristic of Not-self (SN 22.59)