In the Katha Upanisad (1.2.18) we find passage, which reads similar to BG 2.20:
na jayate mriyate va vipascin
nayam kutascin na babhuva kascit
ajo nityah sasvato 'yam purano
na hanyate hanyamane sarire
The meaning and purport of this verse is the same as in the Bhagavad-gita, but here in this verse there is one special word, vipascit, which means learned or with knowledge.
The knowledgeable Soul
The soul is full of knowledge, or full always with consciousness. Therefore, consciousness is the symptom of the soul. Even if one does not find the soul within the heart, where he is situated, one can still understand the presence of the soul simply by the presence of consciousness. Sometimes we do not find the sun in the sky owing to clouds, or for some other reason, but the light of the sun is always there, and we are convinced that it is therefore daytime. As soon as there is a little light in the sky early in the morning, we can understand that the sun is in the sky. Similarly, since there is some consciousness in all bodies -- whether man or animal -- we can understand the presence of the soul. This consciousness of the soul is, however, different from the consciousness of the Supreme because the supreme consciousness is all-knowledge -- past, present and future.
The consciousness of the individual soul is prone to be forgetful. When he is forgetful of his real nature, he obtains education and enlightenment from the superior lessons of Krishna. But Krishna is not like the forgetful soul. If so, Krishna's teachings of Bhagavad-gita would be useless. - Bg 2.20
Yours in service of Srila Prabhupada
Dr Jagadeesh TG
The lotus feet of Srila Prabhupada