कार्यकारणकर्तृत्वे हेतुः प्रकृतिरुच्यते।पुरुषः सुखदुःखानां भोक्तृत्वे हेतुरुच्यते॥
kārya-kāraṇa-kartṛtve hetuḥ prakṛtir ucyatepuruṣaḥ sukha-duḥkhānāṃ bhoktṛtve hetur ucyate
Word by word
The verse says
Nature is said to be the cause in the matter of doing — of what is done and the means of doing it. The person is said to be the cause in the experiencing of pleasure and pain.
In plain words
A clean division, and it is not the one people expect.
Nature does the doing. The person does the undergoing.
So the part of you that acts is on the field side, and the part that feels it is not.
Where you've felt this · at work
A machine in a workshop stamps out the same part four thousand times a day. It does all the work. It does not mind. The man who minds — about the noise, the quota, whether this is what he wanted — does none of the stamping.
Going deeper
Separating the doer from the experiencer is the most counter-intuitive claim in the chapter, and it is the hinge of everything after it.
We treat acting and experiencing as the same person's activity: I did it, I felt it. The verse splits them. Everything that can be described as an action — including the deciding, since buddhi was listed as part of the field — belongs to nature. What remains on the other side is the undergoing.
That sounds as if it removes responsibility, and verse 30 will state it even more sharply.
It does not, for a reason the chapter has already built in. Responsibility, in this book, attaches to how action is done and what it is for — the subject of eleven previous chapters. The claim here is narrower: the sense that you are the originating author of your own activity does not survive examination, and most of the suffering in that activity comes from the authorship rather than the work.