These three sutras are important:
3.1.1:

Om! Which is It that we worship as this Self? Which of the two is the Self? Is It that by which one sees, and by which one hears; also, by which one smells odor, and by which one utters speech, and by which one tastes the sweet or sour?

3.1.2:

It is this heart (intellect) and this mind that were stated earlier. It is sentience, rulership, secular knowledge, presence of mind, retentiveness, sense–perception, fortitude, thinking, genius, mental suffering, memory, ascertainment, resolution, life–activities, hankering, passion and such others. All these are verily the names of Consciousness.
3.1.3:

This One is (the inferior) Brahman; this is Indra, this is Prajapati; this is all these gods; and this is these five elements, viz, earth, air, space, water, fire; and this is all these (big creatures), together with the tiny ones, that the procreators of others and referable in pairs – to wit, those that are born of eggs, of wombs, of moisture, and of the earth, viz, horses, cattle, men, elephants, and all those creatures that there are which move or fly and those which do not move. All these are impelled by Consciousness, all these have Consciousness as the giver of their reality; the universe has Consciousness as its eye, and Consciousness is its end. Consciousness is Brahman.

These sutras are very important. My understanding of these sutras is that, the first sutra asks, which is that that we worship as the Self? Then it asks, is it that by which we see, hear, etc; that is, is it our consciousness? Is our consciousness the Brahman (or not)?
The next sutra gives the various aspects of consciousness like sentience, rulership, etc. and names all the aspects. Then it says, all these are verily the names of consciousness.
The third sutra is the defining sutra, because it answers the first two. It says, all these are the inferior Brahman. All the various aspects of consciousness, and besides this, the gods like Prajapati, all the creatures, from big ones like horses and elephants to small worms, etc. and including humans are all inferior Brahman.
In this sutra, virtually all of creation ranging upto gods is sought to be included. Then it says, all these have Consciousness as the giver of their reality. Here, Brahman is called Consciousness, which is also done in other sutras, and it is said, that it is this Brahman–Consciousness which is the giver of their reality. Thus Brahman is the giver of reality of all these aspects of creation, all living creatures from worms to the gods including humans, and also all non–living bodies since it also includes earth, air, space, water and fire, which covers non–living bodies.
So here in this important sutra, we have Consciousness–Brahman supporting our individual consciousness in all its aspects like sentience, rulership, etc. So this sutra supports the view of Brahman–Consciousness being like a Consciousness field and our own consciousness like a whirlpool in this field. Or we may term it as an Information field and information flux.

Mundaka Upanishad

This Upanishad contains an interesting analogy of our human soul and Brahman as two birds sitting on the same tree. This was a particular favorite of Vivekananda.
3.1.1:

Two birds are ever associated and have similar names, cling to the same tree, of these one eats the fruits of divergent tastes, and the other looks on without eating.

3.1.2:

On the same tree, the individual soul remains drowned (ie, stuck), as it were; and so it moans, being worried by its impotence, when it sees thus the other, the adored Lord, and His glory, then it becomes liberated from sorrow.

3.1.3:

When the seer sees the Purusa — the golden hued, creator, lord, and the source of the inferior Brahman — then the illumined one completely shakes off both the merit and demerit, becomes taintless, and attains absolute equality.

Here, in these verses, there is an interesting use of the word ‘purusa’. In the second sutra, the word ‘purusa’ is used to mean the individual soul, as in ‘the individual soul remains drowned...’. From the context it is clear that it is the individual soul is meant by purusa in this sutra. But in the third sutra, Purusa again undoubtedly stands for the Brahman itself, as it is said, ‘..the source of the inferior Brahman’. Here there is no clear definition of the individual soul. Commentators like Sankaracharya take the individual soul here, the purusa in the second sutra, to mean an independent soul. But this sutra works equally well, and perhaps better, if we take the purusa not to mean a soul as such but our individual consciousness itself. It can be taken to mean either, both an independent soul or the individual consciousness.
3.1.8:

Within the heart in the body, where the vital force has entered in five forms, is this subtle Self, to be realized through that intelligence by which is pervaded the entire mind as well as the motor and sensory organs of all the creatures.. And It is to be known in the mind, which having become purified, this Self reveals Itself distinctly.

Here again, the Self is Brahman itself and not an independent soul. And the rest of the sutra also confirms this.

Prasna Upanishad

4.9:

..And this one, the seer, feeler, hearer, smeller, taster, thinker, ascertainer, doer — the Purusa (pervading the body and organs) who is a knower by nature. He becomes wholly established in the supreme, immutable Self.

Here, the purport of the word ‘Purusa’ appears to be an independent soul. But we can as easily take the word Purusa to stand for individual consciousness and not an independent soul. In many other such sutras in the Upanishads, the word Purusa can be taken to mean both an independent soul or consciousness itself. An independent soul is the more traditional meaning, but these sutras work as well if we take it to mean consciousness. In the context in which this sutra occurs also in the prasna Upanishad, where the previous sutras discussed mainly the mind, we can infer that Purusa here stands for the mind itself.



Thus, through these sutras, we find that the overwhelming message of the Upanishads is that there is no separate entity as an independent soul apart from the mind–body at one level and the absolute Brahman at the other level. It is the Brahman itself which supports our existence and thus said to exist in our heart. The soul is nothing but our consciousness itself. Some parts of the Upanishads do have a message of an independent sul, but this is seen to be only due to the necessity to give support to ideas of reincarnation. Even here, we find that the Upanishads try to emphasize very strongly that this soul is still nothing but the Absolute Brahman itself. If we discard the idea of reincarnation, we need not take these verses as important ones, and instead see them from their historical and cultural paerspective.
There is no doubt that the majority of the Upanishads do not describe an independent soul, and this main philosophy of the Upanishads conforms well with a modern understanding of Advaita and strengthens it as a logical and scientific philosophy which can be accepted by rational minds.END=NAM MO SAKYAMUNI BUDDHA.( 3 TIMES ).WORLD VIETNAMESE BUDDHIST ORDER=VIETNAMESE BUDDHIST NUN=GOLDEN LOTUS MONASTERY=THE EIGHTFOLD PATH.THICH CHAN TANH.THE MIND OF ENLIGHTMENT.AUSTRALIA,SYDNEY.7/1/2015.