Chapter IV · Śloka 18
कर्मण्यकर्म यः पश्येदकर्मणि च कर्म यः ।
स बुद्धिमान्मनुष्येषु स युक्तः कृत्स्नकर्मकृत् ॥
स बुद्धिमान्मनुष्येषु स युक्तः कृत्स्नकर्मकृत् ॥
karmaṇy akarma yaḥ paśyed akarmaṇi ca karma yaḥ |
sa buddhimān manuṣyeṣu sa yuktaḥ kṛtsna-karma-kṛt ||
sa buddhimān manuṣyeṣu sa yuktaḥ kṛtsna-karma-kṛt ||
One who sees inaction in action, and action in inaction, is wise among men; such a one is a yogī and a doer of all action.
This is the pivot-verse of chapter IV: the liberated agent acts fully in the world yet inwardly abides as the uninvolved witness, and the apparent non-doer is actually engaged through intention. Seeing through the action-inaction duality is the mark of wisdom.