All actions are carried out in every way by the gunas of nature; but one whose self is deluded by ego thinks, "I am the doer."
Explanation
Krishna is explaining to Arjuna why action does not truly belong to the ego. The gunas (guṇa) are the three strands of nature: clarity (sattva), passion (rajas) and inertia (tamas). Here, the person who identifies with the ego-sense (ahaṅkāra) takes credit for deeds that nature's forces are producing. Traditions differ on emphasis: Advaita stresses the self as a witness, while devotional schools stress that the real doer is ultimately God.