BG 13.18

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.

13.18. This is the Light even of [all] the lights, [and] is stated to be beyond darkness; It is to be known by [the above] knowledge; It is to be attained [only] by knowledge; and It distinctly remains in the heart of all.

13.18 It is the Light of lights, beyond the reach of darkness; the Wisdom, the only thing that is worth knowing or that wisdom can teach; the Presence in the hearts of all.

13.13-18 Jneyam etc. upto visthitam. Beginningless is the Supreme Brahman : by means of the attributes (descriptions) like these, [the Bhagavat] describes the Brahman as being not separate from the Supreme Consciousness (or action) expressed in every utterance and [thus] gracing [the seeker] to infer his [or Its] own nature. These attributes however have already been explained. Hence what is the use of a fruitless repetition ?

13.18 This (self) alone is the 'light' which illuminates things like the sun, a lamp, a gem etc. It is knowledge alone in the form of the effulgence of the self which illuminates a lamp, the sun etc. But a lamp etc., dispel the darkness that intervenes between the sense of sight and its subject. Their illuminating power is limited to this extent. This is said to be beyond Tamas (darkness). The term Tamas denotes Prakrti in its subtle state. The meaning is that the self transcends Prakrti. Therefore, It is to be comprehended as knowledge, i.e., to be understood as of the form of knowledge. It is attainable by means of knowledge - such as modesty etc., already described. It is present in the heart of all, i.e., It is specially settled, or present in the heart of all beings like men etc.

13.18 Tat, that Knowable; is the jyotih, Light; api, even; jyotisam, of the lights-of the sun etc. For the lights like the sun etc. shine because they are enkindled by the light of consciousness of the Self, as is known from Upanisadic texts like, 'Illumined by whose light the sun shines' (Tai. Br. 3.12.9.7), 'By Its light all this shines variously' (Sv. 6.14), and from the Smrti also, as here (in the Gita) itself: 'That light in the sun৷৷.' (15.12), etc. It is ucyate, spoken of as; param, beyond, untouched by; tamasah, darkness; ignorance. For cheering up anyone who may become disheartened by thinking that Knowledge etc. is difficult to attain, the Lord says: It is jnanam, Knowledge-humility etc. (verse 7, etc.); jneyam, the Knowable, which has been spoken of in, 'I shall speak of that which is to be known' (12); and jnana-gamyam, the Known. The Knowable itself is referred to as jnanagamyam, when after being known, It becomes the result of Knowledge. But when It is an object to be known, It is called jneyam. All these three which are such, visthitam, specially exist; hrdi, in the hearts, in the intellects; sarvasya, of all, of all creatures. For these three are, indeed, perceived there. This verse is begun for concluding the topic under discussion:

13.18 The light of all lights, this is said to be beyond Tamas (darkness). It is known to be knowledge. It is to be attained by knowledge. It is present in the heart of all.

13.18 That is the Light even of the lights; It is spoken of as beyond darkness. It is Knowledge, the Knowable, and the Known. It exists specially [A variant reading is dhisthitam.-Tr.] in the hearts of all.