Verse 3.18.3

वागेव ब्रह्मणश्चतुर्थः पादः सोऽग्निना ज्योतिषा भाति च तपति च भाति च तपति च कीर्त्या यशसा ब्रह्मवर्चसेन य एवं वेद ॥ ३.१८.३ ॥

vāgeva brahmaṇaścaturthaḥ pādaḥ so'gninā jyotiṣā bhāti ca tapati ca bhāti ca tapati ca kīrtyā yaśasā brahmavarcasena ya evaṃ veda || 3.18.3 ||

3. Vāk is one of the four feet of Brahman [as the mind]. It shines in the light of fire and also radiates heat. He who knows this shines and radiates warmth with his good work, with his fame, and with the radiance he acquires from leading a scholarly and disciplined life.

Word-for-word explanation:

Vāk eva brahmaṇaḥ caturthaḥ pādaḥ, speech is the fourth foot of [manomaya] Brahman; agninā jyotiṣā, by the light it gets from fire; saḥ bhāti ca tapati ca, it shines and gives heat; yaḥ evam veda, he who knows thus; bhāti ca tapati ca, shines and radiates warmth; kīrtyā, through good work; yaśasā, through fame; brahmavarcasena, through the radiance that comes from a scholarly and disciplined life.

Commentary:

Animals move where they want by their feet. Similarly, people find their way to Brahman by hearing someone speak about it. This is why the organ of speech is called one of the four feet of Brahman.

Fire gives both light and heat. And because fire is the presiding deity of speech, speech also is said to give light and heat. A person who can speak well acquires fame and popularity; It is as if he has done much good work, or has given away much money in charity, or is a great scholar and has lived a disciplined life. Such a person has a shining personality and radiates heat (i.e., energy).