The Buddha teaches about dependent co-arising and the phenomena arisen from dependent co-arising. A noble disciple who has thoroughly seen this with right wisdom will no longer be doubtful about who they were in the past, who they will be in the future, or who they are in the present.
Paccaya sutta - Dependence
At Sāvatthi.
"I will teach you, bhikkhus, about dependent co-arising and the phenomena arisen from dependent co-arising. Listen to this and pay close attention, I will speak."
"Yes, venerable sir," those bhikkhus replied to the Blessed One. The Blessed One said:
"And what, bhikkhus, is dependent co-arising? With dependence on birth, bhikkhus, there is aging and death. Whether or not there is the arising of Tathāgatas, this principle remains — this stability of phenomena, this invariance of phenomena, this general law of conditionality. This, a Tathāgata awakens to and breaks through to. Having fully awakened to and broken through to it, he points it out, teaches it, declares it, establishes it, clarifies it, analyzes it, and reveals it. And he says: 'See, bhikkhus, with dependence on birth, there is aging and death.'
With dependence on existence, there is birth ... with dependence on clinging, there is existence ... with dependence on craving, there is clinging ... with dependence on sensation, there is craving ... with dependence on contact, there is sensation ... with dependence on the six sense bases, there is contact ... with dependence on name and form, there are the six sense bases ... with dependence on consciousness, there is name and form ... with dependence on volitional formations, there is consciousness ... with dependence on ignorance, there are volitional formations. Whether or not there is the arising of Tathāgatas, this principle remains — this stability of phenomena, this invariance of phenomena, this dependence. This, a Tathāgata awakens to and breaks through to. Having fully awakened to and broken through to it, he points it out, teaches it, declares it, establishes it, clarifies it, analyzes it, and reveals it. And he says: 'See, bhikkhus, with dependence on ignorance, there are volitional formations.' Thus, bhikkhus, the actuality in this, the invariability, the not-otherwiseness in dependence — this is called dependent co-arising.
And what, bhikkhus, are the phenomena that arise through dependent co-arising? Aging and death, bhikkhus, are impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to gradual ending.
Birth, bhikkhus, is impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
Existence, bhikkhus, is impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subjecting to ending.
Clinging, bhikkhus, is impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
Craving, bhikkhus, is impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
Sensation, bhikkhus, is impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
Contact, bhikkhus, is impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
The six sense bases, bhikkhus, are impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
Name and form, bhikkhus, are impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
Consciousness, bhikkhus, is impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
Volitional formations, bhikkhus, are impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending.
Ignorance, bhikkhus, is impermanent, conditioned, dependently arisen, subject to wearing away, liable to disintegrate, subject to fading away and subject to ending. These are called, bhikkhus, the phenomena arisen from dependent co-arising.
Bhikkhus, when a noble disciple has thoroughly seen with right wisdom, as it truly is, 'This is dependent co-arising, and these are the phenomena arising from dependent co-arising,' they will no longer run back to the past, wondering, 'Was I in the past? Was I not in the past? What was I in the past? How was I in the past? Having been what, what was I in the past?' Nor will they run forward to the future, wondering, 'Will I be in the future? Will I not be in the future? What will I be in the future? How will I be in the future? Having become what, what will I be in the future?' Nor will they be inwardly doubtful about the present, wondering, 'Am I? Am I not? What am I? How am I? Where has this being come from, and where will it go?'
For what reason? Because, bhikkhus, a noble disciple has thoroughly seen with right wisdom, as it truly is, 'This is dependent co-arising, and these are the phenomena arising from dependent co-arising.'"