Atman or Self aka soul
Upanishads are philosophical works, have have discussed a wide ranging topics in philosophy. They are source of Indian philosophy. Even Buddha drew his ideas from them.
1. Prajapati said: "The Self which is free from sin, free from old age, free from death, free from grief, free from hunger, free from thirst, whose desires come true and whose thoughts come true - That it is which should be searched out, That it is which one should desire to understand. He who has known this Self from the scriptures and a teacher and understood It obtains all the worlds and all desires.
Indra, the chief god came and desired to be instructed about Self.
24.1.144. Chapter VII - The Person in the Eye
4. Prajapati said to them: "The person that is seen in the eye - that is the Self." He further said: "This is immortal, fearless. This is Brahman."
They asked: "Venerable Sir, he who is perceived in the water and he who is perceived in a mirror - which of these is he?"
Prajapati replied: "The same one, indeed, is perceived in all these."
Indra pondered on this and his doubts cropped up.
"As this reflection in the water is well adorned when the body is well adorned, well dressed when the body is well dressed, clean when the body is clean, so this reflection in the water will be blind if the body is blind, one - eyed if the body is one - eyed, crippled if the body is crippled and will perish if the body perishes.
2. "I do not see any good in this doctrine." He returned and expressed his doubts.
3. "So it is Indra," replied Prajapati. "I shall explain the Self to you further.
Prajapati accepted the objections.
24.1.147. Chapter X - The Dream Self
Prajapati next taught:
1 - 2. "He who moves about, exalted, in dreams - this is the Self, this is immortal, fearless. This is Brahman."
Indra thought again, but he had more doubts. So he returned and expressed the same.
"Although this dream self is not blind even if the body is blind, nor do its eyes and nose run when the eyes and nose of the body run; although this self is not affected by the defects of the body,
"Nor killed when it (the body) is killed, nor one - eyed when it is one - eyed - yet they kill it (the dream self), as it were; they chase it, as it were. It becomes conscious of pain, as it were; it weeps, as it were. I do not see any good in this doctrine."
"So it is, Indra," replied Prajapati. "I shall explain the Self further to you."
Then Prajapati said to Indra
24.1.148. Chapter XI - The Self in Dreamless Sleep
1. "When a man is asleep, with senses withdrawn and serene and sees no dream - that is the Self. This is immortal, fearless. This is Brahman."
Indra was not automaton. He thought and found difficulties.
He (Indra) said: "Venerable Sir, in truth it (i.e. the self in dreamless sleep) does not know itself as 'I am it,' nor these other creatures. It has therefore reached utter annihilation, as it were. I do not see any good in this."
3. "So it is, Indra," replied Prajapati. "I shall explain the Self further to you and nothing else.
Then Prajapati said to him:
24.1.149. Chapter XII - The Incorporeal Self
1. "O Indra, this body is mortal, always held by death. It is the abode of the Self which is immortal and incorporeal. The embodied self is the victim of pleasure and pain. So long as one is identified with the body, there is no cessation of pleasure and pain. But neither pleasure nor pain touches one who is not identified with the body.
2 - 3. "The wind is without body; the cloud, lightning and thunder are without body. Now, as these, arising from yonder akasa and reaching the highest light, appear in their own forms,
"So does this serene Being, arising from this body and reaching the Highest Light, appear in His own form. In that state He is the Highest Person. There He moves about, laughing, playing, rejoicing - be it with women, chariots, or relatives, never thinking of the body into which he was born.
"As an animal is attached to a cart, so is the prana (i.e. the conscious self) attached to the body.
4. "When the person in the eye resides in the body, he resides where the organ of sight has entered into the akasa (i.e. the pupil of the eye); the eye is the instrument of seeing. He who is aware of the thought: 'Let me smell this,' he is the Self; the nose is the instrument of smelling. He who is aware of the thought: 'Let me speak,' he is the Self; the tongue is the instrument of speaking. He who is aware of the thought: 'Let me hear,' he is the Self; the ear is the instrument of hearing.
5. "He who is aware of the thought: 'Let me think this,' he is the Self; the mind is his divine eye. He, the Self sees all these desires in the World of Brahman through the divine eye, the mind and rejoices.
6. "The gods meditate on that Self. Therefore all worlds belong to them and all desires. He who knows that Self and understands It obtains all worlds and all desires." Thus said Prajapati, yea, thus said Prajapati.
The book ends with:
Om ! Let my limbs and speech, Prana, eyes, ears, vitality
And all the senses grow in strength.
All existence is the Brahman.
May I never deny Brahman, nor Brahman deny me.
Let there be no denial at all:
Let there be no denial at least from me.
May the virtues that are proclaimed in the Upanishads be in me,
Who am devoted to the Atman; may they reside in me.
Om ! Peace ! Peace ! Peace !
In many of the Upanishads [108 in number], a step by step approach is followed.