Bhagavad Gita Chapter 13, Text 18

Bg 13.18

jyotisam api taj jyotis
tamasah param ucyate
jnanam jneyam jnana-gamyam
hrdi sarvasya visthitam

Word for word: 
jyotisam — in all luminous objects; api — also; tat — that; jyotih — the source of light; tamasah — the darkness; param — beyond; ucyate — is said; jnanam — knowledge; jneyam — to be known; jnana-gamyam — to be approached by knowledge; hrdi — in the heart; sarvasya — of everyone; visthitam — situated.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
He is the source of light in all luminous objects. He is beyond the darkness of matter and is unmanifested. He is knowledge, He is the object of knowledge, and He is the goal of knowledge. He is situated in everyone’s heart.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Supersoul, the Supreme Personality of Godhead, is the source of light in all luminous objects like the sun, moon and stars. In the Vedic literature we find that in the spiritual kingdom there is no need of sun or moon, because the effulgence of the Supreme Lord is there. In the material world that brahma-jyotir, the Lord’s spiritual effulgence, is covered by the mahat-tattva, the material elements; therefore in this material world we require the assistance of sun, moon, electricity, etc., for light. But in the spiritual world there is no need of such things. It is clearly stated in the Vedic literature that because of His luminous effulgence, everything is illuminated. It is clear, therefore, that His situation is not in the material world. He is situated in the spiritual world, which is far, far away in the spiritual sky. That is also confirmed in the Vedic literature. Aditya-varnam tamasah parastat (Svetasvatara Upanisad 3.8). He is just like the sun, eternally luminous, but He is far, far beyond the darkness of this material world.

His knowledge is transcendental. The Vedic literature confirms that Brahman is concentrated transcendental knowledge. To one who is anxious to be transferred to that spiritual world, knowledge is given by the Supreme Lord, who is situated in everyone’s heart. One Vedic mantra (Svetasvatara Upanisad 6.18) says, tam ha devam atma-buddhi-prakasam mumuksur vai saranam aham prapadye. One must surrender unto the Supreme Personality of Godhead if he at all wants liberation. As far as the goal of ultimate knowledge is concerned, it is also confirmed in Vedic literature: tam eva viditvati mrtyum eti. “Only by knowing Him can one surpass the boundary of birth and death.” (Svetasvatara Upanisad 3.8)

He is situated in everyone’s heart as the supreme controller. The Supreme has legs and hands distributed everywhere, and this cannot be said of the individual soul. Therefore that there are two knowers of the field of activity – the individual soul and the Supersoul – must be admitted. One’s hands and legs are distributed locally, but Krishna’s hands and legs are distributed everywhere. This is confirmed in the Svetasvatara Upanisad (3.17): sarvasya prabhum isanam sarvasya saranam brhat. That Supreme Personality of Godhead, Supersoul, is the prabhu, or master, of all living entities; therefore He is the ultimate shelter of all living entities. So there is no denying the fact that the Supreme Supersoul and the individual soul are always different.