Bhagavad Gita Chapter 18, Verse 16

तत्रैवं सति कर्तारमात्मानं केवलं तु यः | पश्यत्यकृतबुद्धित्वान्न स पश्यति दुर्मतिः ||

tatraivaṁ sati kartāram ātmānaṁ kevalaṁ tu yaḥ paśyaty akṛta-buddhitvān na sa paśyati durmatiḥ

In that case, one who, due to lack of understanding, sees the self alone as the doer, that person of poor understanding does not see correctly.

Krishna explains the misunderstanding that arises from not seeing the five causes. 'Tatraivaṁ sati'—in that case (when there are five causes). 'Kartāram ātmānaṁ kevalaṁ tu yaḥ paśyaty akṛta-buddhitvāt'—one who (yaḥ), due to lack of understanding (akṛta-buddhitvāt), sees (paśyati) the self alone (ātmānaṁ kevalam) as the doer (kartāram). 'Na sa paśyati durmatiḥ'—that person of poor understanding (durmatiḥ) does not see correctly (na paśyati). This is the key teaching: when you see only yourself as the doer, ignoring the other four causes (body, instruments, efforts, Supreme), you have poor understanding. You're not seeing the full picture. This misunderstanding leads to ego-based thinking where you take full credit or full blame. True understanding sees all five causes working together. This sets up the teaching about the three kinds of action according to gunas, where the distinction is in the quality of understanding and the nature of the action.