In plain words
The vyaṣṭi becomes the jīva; the samaṣṭi is Īśvara. Both are bound to perish! Why? Because both are only effects! Only the nirguṇa Brahman, the very embodiment of the cause, remains without change, unmoving.
In jīvas too there are the pāramārthika and the prātibhāsika. And the vyāvahārika: so there are three kinds. But these three, like the states of waking, dream and deep sleep, like the waves and the foam in water, are seen on close examination to be of one single form. Apart from the water there are no waves; apart from the waves there is no foam. In the water the waves are imagined; in the wave the foam is imagined. In the same way, from the pāramārthika comes the prātibhāsika, and from the prātibhāsika the vyāvahārika: each is the support of the next. The Kūṭastha, the pāramārthika jīva, is reflected in the prātibhāsika; the prātibhāsika is reflected in the vyāvahārika.
Kūṭastha means the one who undergoes no change in any state whatever. That is what it means. Keeping food, sleep, fear, sex and the rest far from him, he remains established in the state of eternal Ānanda, “Ahaṃ brahmāsmi.”
“What is the duty man must follow to reach so holy a state, to attain such Brahmajñāna?” Everyone must recognise this. For this very purpose they have laid down the fourfold sādhana.
Inquiry into the eternal and the transient; Virāga toward enjoying the fruits of this world and the next; the six treasures, śama, dama and the rest; and mumukṣutva. These four together are called the fourfold sādhana. To study the Śruti, the Smṛti, the Śāstras, the Itihāsas, the Purāṇas and the rest, to grasp the eternal truths in them, and to hold the antaḥkaraṇa steady in your own experience in the state of “Brahma satyaṃ jaganmithya”: that alone is the fulfilment of this inquiry!