nityaḥ nityānāṃ ca yaḥ anuśāyī

nityaḥ (eternal) yaḥ (who) nityānāṃ (of the eternal ones) ca (and) anuśāyī (is the overseer)

Advaita Vedānta (Ādi Śaṅkarācārya): The verse 'nityo nityānāṃ ca yo 'nuśāyī' underscores the concept of the eternal companion that permeates all eternally existing things. According to Shankara's commentary on the Mundakopanishad, this verse points to the ultimate reality, Brahman, which is the essence of all that exists. The term 'nityānām' refers to the permanent selves (ātman) that are beyond the transient world. The 'anuśāyī' or companion of these permanent selves is Brahman itself, indicating the non-dual nature of reality where Brahman and Ātman are ultimately one. This interpretation aligns with Shankara's reasoning in his commentary on the Taittiriya Upanishad, where he argues for the identity of Brahman and Ātman, emphasizing that the ultimate reality is beyond all duality and plurality.

Vaiṣṇava tradition (Rāmānujācārya / Madhvācārya): In the Vaiṣṇava tradition, the verse 'nityo nityānāṃ ca yo 'nuśāyī' is understood as referring to the relationship between the individual self (jīva) and the Supreme Lord (Īśvara). Rāmānujācārya, in his commentary on the Upanishads, interpreting this verse, highlights the dependence of the jīva on Īśvara, where the Supreme Lord is the constant companion and sustainer of all eternal beings. This interpretation emphasizes the concept of 'śeṣa-śai' or the servant-master relationship, where the jīva is in a state of eternal servitude to Īśvara, underscoring the devotional aspect of the Vaiṣṇava tradition. Madhvācārya, on the other hand, would likely interpret this verse in the context of his Dvaita Vedanta, emphasizing the real distinction between the independent Īśvara and the dependent jīva, yet affirming their eternal relationship.

Neo-Vedānta (Swami Vivekānanda / S. Rādhākrishnan): Swami Vivekānanda, interpreting this verse in the context of Neo-Vedānta, emphasizes the universal and eternal essence that accompanies all eternal beings. He sees this companion as the 'Self' or 'Ātman', highlighting the unity and interconnectedness of all existence. In his lectures and writings, Vivekānanda often drew upon such Upanishadic verses to illustrate the practical and universal applications of Vedāntic philosophy, encouraging individuals to seek and realize this eternal companion within themselves. Similarly, S. Radhakrishnan, in his philosophical works, would interpret this verse as signifying the enduring and immutable aspect of human consciousness, underscoring the importance of realizing one's true nature as part of the universal whole, thereby achieving a state of freedom and peace in contemporary life.

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