In plain words
Animals, birds and beasts all go through these four states. And then: jantūnāṃ narajanma durlabham. What is the special meaning in that word? The speech that is in man is very important. It is made of radiance. Manas is made of food. And the prāṇa is made of essence. To unite speech, which is made of radiance, and the body, which is made of food, with the prāṇa, which is made of essence: this is its highest goal.
The Ātma is nothing but the form in which these three, speech, Manas and prāṇa, are blended. To understand this vast principle of the Ātma, we must make inquiries of certain kinds. For this there are three states. Jāgrat, svapna, suṣupti. What is jāgrat? Being awake, seeing, hearing, experiencing, speaking in this world fashioned of what is seen: this principle made up of the five senses is the outer world. Because it is the subtle form of the principle of the cosmos, it has another name, Viśva. This Viśva is made up of twenty-four principles. The five organs of action, the five tanmātras, the five organs of knowledge, the five prāṇas: these make twenty. Manas, Buddhi, citta and Ahaṅkāra: these four are the antaḥkaraṇa. What is made of all these is the outer world. This itself they called Viśva. This is the form of the waking state.
The second is the dream state. It is joined with only a few of the senses; the outer organs of action are of no use to it at all. By itself it wills everything and creates it. It is made up of these four: Manas, Buddhi, citta and Ahaṅkāra. In this condition it takes on a form made of radiance. That is why they named the dream state Taijasa.
And then the causal state, suṣupti. ‘Su’ means good. Ṣupti means sleep. Suṣupti means good, deep sleep. In this deep sleep only Prajñā remains. That is why they named suṣupti Prājña.
So Viśva, Taijasa and Prājña, all three, are only different names of the Ātma. That is why in the waking state the Ātma takes on the form of the cosmos. In the dream state the Ātma takes on a form made of radiance. In deep sleep, in suṣupti, it takes on the form of Prajñā. That is why they said, Prajñānaṃ Brahma.
What does this Prajñāna mean? Today we come to know knowledge of many kinds: Jñāna, Vijñāna, sujñāna. Jñāna, Vijñāna, sujñāna, ajñāna, thought, will, intelligence: all these are different names of the Ātma.