na tu eva idam agre atho idānīṃ na tu eva idam upaadhāvataḥ

na tu (not indeed) eva (indeed) idam (this) agre (at first) atho (then) idānīṃ (now) na tu (not indeed) eva (indeed) idam (this) upadhāvataḥ (runs after)

Advaita Vedānta (Ādi Śaṅkarācārya): In his commentary on the Kaṭhopaniṣad, Śaṅkarācārya interprets this verse as pointing to the timeless and unchanging nature of the Self (Ātman). He argues that the phrase 'na tv evedaṃ agre' indicates that the Self is not something that comes into existence at the beginning of creation, and 'na tv evedaṃ idānīṃ' suggests that it is not something that exists in the present moment as a separate entity. Furthermore, 'na tv evedaṃ upadhāvataḥ' implies that the Self will not come into existence in the future. This reasoning underscores the non-dual reading of the verse, emphasizing that the ultimate reality, Brahman, is beyond the constraints of time and space, and is, in fact, the very essence of the Self. Śaṅkarācārya's commentary on the Taittirīya Upaniṣad (2.1.1) also supports this view, where he states that the Self is 'satyam, jñānam, anantam' (true, conscious, and infinite), highlighting the identity of Brahman and Ātman.

Vaiṣṇava tradition (Rāmānujācārya): Rāmānujācārya, in his commentary on the Kaṭhopaniṣad, views this verse as emphasizing the distinction between the individual self (jīva) and the Supreme Self (Īśvara). The phrase 'na tv evedaṃ agre' suggests that the individual self is not autonomous at the beginning, and 'na tv evedaṃ idānīṃ' implies that it is not independent in the present. The phrase 'na tv evedaṃ upadhāvataḥ' indicates that the individual self will not attain liberation through its own efforts in the future. Rāmānujācārya's commentary highlights the need for devotion to Īśvara, who is the ultimate reality, and the source of the individual self's existence. This devotional reading underscores the relationship between the jīva and Īśvara, where the individual self seeks refuge in the Supreme Self to attain liberation. In his Śrī Bhāṣya, Rāmānujācārya also discusses the concept of 'aprthak-siddhi,' which refers to the inherent and inseparable nature of the individual self and the Supreme Self, supporting this interpretation.

Neo-Vedānta (Swami Vivekānanda): Swami Vivekānanda, in his lectures and writings, often emphasized the practical application of Upaniṣadic verses like this one. He viewed this verse as highlighting the timeless and universal nature of the Self, which is beyond the limitations of time and space. The phrase 'na tv evedaṃ agre' suggests that the Self is not bound by the past, and 'na tv evedaṃ idānīṃ' implies that it is not restricted by the present. The phrase 'na tv evedaṃ upadhāvataḥ' indicates that the Self will not be limited by the future. In his lecture 'The Real Nature of the Self,' Vivekānanda states that the Self is 'the essence of knowledge, the essence of existence, and the essence of bliss,' which is in line with the idea that the Self is beyond the constraints of time. This universal and practical application of the verse is also reflected in S. Radhakrishnan's writings, where he emphasizes the importance of realizing the true nature of the Self in everyday life, leading to a more fulfilling and meaningful existence.

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