On the insistent questioning of the highly determined Naciketas, Lord Yama had no alternative but to reveal the secret code to ending the transient mortal world and realizing the “immortality” that one actually and already is. It is not some thing new that one acquires. It is prAptasya prAptiH (प्राप्तस्य प्राप्ति:). Or, as kaTha says at 2.5.1, विमुक्तश्च विमुच्यते (i.e. becoming freed, one becomes emancipated. In other words, he does not take up a body again).
Shankara explains it in his own inimitable way unpacking the involved intricacies in simple words. He writes in his commentary at 1.3.14, kaTha in the following way:
एवं पुरुषे आत्मनि सर्वं प्रविलाप्य नामरूपकर्मत्रयं यन्मिथ्याज्ञानविजृम्भितं क्रियाकारकफललक्षणं स्वात्मयाथात्म्यज्ञानेन मरीच्युदकरज्जुसर्पगगनमलानीव मरीचिरज्जुगगनस्वरूपदर्शनेनैव स्वस्थः प्रशान्तः कृतकृत्यो भवति यतः , अतस्तद्दर्शनार्थमनाद्यविद्याप्रसुप्ताः उत्तिष्ठत हे जन्तवः | — Shankara at 1.3.14, kaTha Upanishad.
Meaning: Having thus melted** (pravilApya) into the puruSha, the Atman, all the three, i.e. name, form and karma, which are produced by false knowledge and are of the nature of action, agents and fruits, by a Knowledge of the true nature of the Atman, as the water in the mirage, the serpent in the rope and the color of the space, disappear by seeing the true nature of the mirage, rope and the space, one becomes free from anxiety and calm, his purpose accomplished. Therefore, to know that, Arise, Oh, living beings sleeping in beginning-less ignorance, i.e., turn towards the acquisition of the Knowledge of the Atman; and Awake, i.e., put an end to the sleep of ignorance, horrible in form and the seed of all misery. (Translation: S. Sitarama Sastri, 1923).
[** – The author used the word “merged” which is replaced by me as “melted” in order to convey the true philosophical sense of the “process of pravilApana.” The accent on “disappear” is also by me.]
Shankara takes up the issue of dissolving (melting or sublating) the apparent world for a detailed discussion at 3.2.21, sUtra bhAShya. He asks the Questions:
कोऽयं प्रपञ्चप्रविलयो नाम ?
किमग्निप्रतापसम्पर्कात् घृतकाठिन्यप्रविलय इव प्रपञ्चप्रविलयः कर्तव्यः?
आहोस्विदेकस्मिंश्चन्द्रे तिमिरकृतानेकचन्द्रप्रपञ्चवत् अविद्याकृतो ब्रह्मणि नामरूपप्रपञ्चो विद्यया प्रविलापयितव्यः ?
Meaning: “What is meant by this sublation of the universe of manifestations?
Is the world to be annihilated like the destruction of the solidity of ghee ( solidified melted butter) by contact with fire ?
Or is it that the world of name and form, created in brahman by nescience like many moons created in the moon by the eye-disease called timira (similar to cataract), has to be destroyed through knowledge?” (Translation: Swami Gambhirananda).
And he himself gracefully answers leaving no scope for further doubt. His Answer is:
तत्र यदि तावद्विद्यमानोऽयं प्रपञ्चः देहादिलक्षण आध्यात्मिकः बाह्यश्च पृथिव्यादिलक्षणः प्रविलापयितव्य इत्युच्यते, स पुरुषमात्रेणाशक्यः प्रविलापयितुमिति तत्प्रविलयोपदेशोऽशक्यविषय एव स्यात् । एकेन च आदिमुक्तेन पृथिव्यादिप्रविलयः कृत इति इदानीं पृथिव्यादिशून्यं जगदभविष्यत् । अथ अविद्याध्यस्तो ब्रह्मण्येकस्मिन् अयं प्रपञ्चो विद्यया प्रविलाप्यत इति ब्रूयात् , ततो ब्रह्मैव अविद्याध्यस्तप्रपञ्चप्रत्याख्यानेन आवेदयितव्यम् — ‘एकमेवाद्वितीयं ब्रह्म’ ‘तत्सत्यꣳ स आत्मा तत्त्वमसि’ (छा. उ. ६ । ८ । ७) इति | तस्मिन्नावेदिते, विद्या स्वयमेवोत्पद्यते ; तया च अविद्या बाध्यते, ततश्च अविद्याध्यस्तः सकलोऽयं नामरूपप्रपञ्चः स्वप्नप्रपञ्चवत् प्रविलीयते — अनावेदिते तु ब्रह्मणि ‘ब्रह्मविज्ञानं कुरु प्रपञ्चप्रविलयं च’ इति शतकृत्वोऽप्युक्ते न ब्रह्मविज्ञानं प्रपञ्चप्रविलयो वा जायते ।
“Now if it be said that this existing universe of manifestations, consisting of the body etc. on the corporeal plane and externally of the earth etc., is to be annihilated, that is a task impossible for any man, and hence the instruction about its extirpation is meaningless.
Moreover, (even supposing that such a thing is possible, then) the universe, including the earth etc., having been annihilated by the first man who got liberation, the present universe should have been devoid of the earth etc.
Again, if it be said that this universe of manifestations superimposed on the one brahman alone through ignorance has to be sublated by enlightenment, then it is brahman Itself that has to be presented through a denial of the manifestation superimposed by ignorance by saying, “brahman is one without a second” (chAndogya, 6.2.1), “That is Truth, That is the Self, That thou art (Svetaketu)” (chAndogya 6.8.7-16). When brahman is taught thus, Knowledge dawns automatically, and by that Knowledge ignorance is removed.
As a result of that, this whole manifestation of name and form, superimposed by ignorance, vanishes away like things seen in a dream.
But unless brahman is (first) taught (by scripture etc.), neither does the Knowledge of brahman dawn nor is the universe sublated even though the instruction, “Know brahman, sublate the world”, be imparted a hundred times.” (Translation: Swami Gambhirananda).
It is also helpful here to recall Shankara’s observations at 2.4.13, brihadAraNyaka:
सा कार्यकरणसङ्घातोपाधौ प्रविलापिते नश्यति, हेत्वभावात् , उदकाद्याधारनाशादिव चन्द्रादिप्रतिबिम्बः तन्निमित्तश्च प्रकाशादिः | —Shankara at 2.4.13, brihadAraNyaka Upanishad.
Meaning: When the individual existence of the self that is superimposed by ignorance and is connected with the body and organs is destroyed by Knowledge, the particular consciousness connected with the body etc., consisting of a false notion, is destroyed on the destruction of the limiting adjuncts of the body and organs, for they are deprived of their cause, just as the reflections of the moon etc., and their effects, the light and so forth, vanish when the water and the like, which form their support, are gone. (Translation: Swami Madhavananda).
The very subtle point that the Advaita makes is that one need not and, in fact, should not try to dabble with or meddle with the “perceived” (i.e. the world) but correct one’s own vision (दृष्टि – dRiShTi). Just like correcting the cataract vision will make the blurred double-moon disappear, the multiplicity, which in Reality has never been there at all, will disappear once the “particular” consciousness ends. Then the Universal Consciousness will automatically shine.