Verse 3.1.4

प्रणो ह्येष यः सर्वभूतैर्विभाति विजानन्विद्वान्भवते नातिवादी ।
आत्मक्रीड आत्मरतिः क्रियावानेष ब्रह्मविदां वरिष्ठः ॥ ४ ॥

praṇo hyeṣa yaḥ sarvabhūtairvibhāti vijānanvidvānbhavate nātivādī |
ātmakrīḍa ātmaratiḥ kriyāvāneṣa brahmavidāṃ variṣṭhaḥ || 4 ||

4. This is, indeed. Prana, i.e., Isvara, shining variously with all living beings. Knowing him, the wise man becomes not a talker regarding anything else. Sporting in self, delighted in self and doing nets (enjoined), this man is the best of those who know the Brahman.

Shankara’s Commentary:

Com.—Again this Isvara is the prana of prana. This, now treated of. ‘All living things,’ from the Brahman down to the worm. The instrumental case in “Sarvabhutaih” has the force of “thus become.” The meaning is ‘existing in all living things, i.e., the atman of all.’ Vibhati, shines variously. The man of knowledge who directly realises Him who is in all things as his own Atman and thinks “I am he” does not become an ativadin. merely by the knowledge of the import of the mahavakya. Ativadi, means one whose nature is to talk of all other things more, when he sees that all is the Atman and nothing else exists; how then could he talk of anything else. It is only where one sees anything else, he could well talk of that; but this man of knowledge sees, hears and knows none other than the Atman; so. he is not a talker of anything else. Again, he is an atmakridah, i.e., one whose sport is within his own Atman and not elsewhere, such as son. wife, etc. Similarly, he is atmaratih, i.e., one who delights or revels in his own Atman. The term krida or play requires some external help. But delight or revelling does not require any external help but indicates merely the attachment to an external object. This is the distinction. Similarly, kriyavan, i.e., one whose activity consists in knowledge, meditation, freedom from desire, etc. But if the reading he “atmaratikriyavan” (a compound) the meaning is he whose activity is mere delight in man; as between the Bahuvrihi and the “matup” ending, one is sufficient without the other; but some contend that the single compound, denotes a combination of both karma, i.e., Agnihotra and the rest, and the knowledge of Brahman. It is not possible for one to he playing with external objects and at the same time to he delighted in self. It is only the man who has turned away from external activity that becomes delighted in self; for, external activity and delighted in itself are opposed to each other; for, it is not possible that darkness and light could exist in the same place. Therefore, the statement that a combination of karma and knowledge is inculcated by this text is certainly the prattle of the ignorant. This also follows from the Srutis “Leave off from all other speech” and “by renunciation of Karma, etc.” Therefore, he alone is “kriyavan” whose activity consists in knowledge, meditation, etc., and who is a Sanyasin not transgressing the limits of prohibitory injunctions. Such a man as is not an ativadin, as sports in his own self, as delights in himself and as is a Kriyavan (whose activity is aforesaid) is the first among all the knowers of Brahman.