Kena 2.4 Pratibodha-viditam(Brahman is known through every cognition)

pratibodhaviditaṁ matam amṛtatvaṁ hi vindate, ātmanā vindate vīryaṁ vidyayā vindate’mṛtam. (2.4) 

pratibodha-viditam – known through every cognition; matam – known; amṛtatvam – immortality; hi – indeed; vindate – gains; ātmanā – by oneself; vindate – gains; vīryam – capacity; vidyayā – by knowledge; vindate – gains; amṛtam – immortality

~Kena 2.4 (tr-Dayananda

Brahman is known through every cognition. One, indeed, gains immortality (from that cognition). One gains the capacity (to know) by oneself. (Thereafter) one gains immortality by knowledge.

~Kena 2.4 (tr-Dayananda)

Indeed, he convincingly attains immortality, who intuits It in and through every modification of the mind. Through the Ᾱtman he obtains real strength, and through knowledge, immortality.

~Kena 2.4 (tr-Chinmayananda)

Pratibodha-viditam, known with reference to each state of intelligence. By the word bodha are meant the cognitions acquired through the intellect. The Self, that encompasses all ideas as Its objects, is known in relation to all these ideas. Being the witness of all cognitions, and by nature nothing but the power of Consciousness, the Self is indicated by the cognitions themselves, in the midst of cognitions, as pervading (all of) them. There is no other door to Its awareness. Therefore when Brahman is known as the innermost Self (i.e. witness) of cognitions, then is It matam, known, that is to say, then there is Its complete realization. Only by accepting Brahman as the witness of all cognitions can it be established that It is by nature a witness that is not subject to growth and decay, and is eternal, pure in essence, the Self, unconditioned, and one in all beings...

~Shankara's commentary on Kena 2.4 (tr-Gambhirananda)

How, again, is immortality attained through the aforesaid knowledge of the Self? This is being answered. Atmanā, through one's own Self; vindate, (one) attains; vīryam, strength, capacity. The strength got from wealth, friend, incantation, medicine, asuterity, or Yoga cannot conquer death, for it is produced by impermanent things. But the strength, consequent on the knowledge of the Self, is acquired through the Self alone and not through anything else. Thus, since the strength resulting from the knowledge of the Self is independent of any means of acquisition, that strength alone is able to conquer death. Since the strength produced by the knowledge of the Self is thus attained through the Self, therefore, vidyayā, through knowledge about the Self; (one) vindate, attains; amṛtam, immortality.

~Shankara's commentary on Kena 2.4 continued (tr-Gambhirananda)

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In other words, the witness (pure consciousness) is intuited within ordinary cognition, and it is that witness (aka atman) which alone provides the strength for knowledge (brahmavidya) and thus self-realization (immortality).