Sanskrit
कार्यकारणकर्तृत्वे हेतुः प्रकृतिरुच्यते । पुरुषः सुखदुःखानां भोक्तृत्वे हेतुरुच्यते ॥
kāryakāraṇakartṛtve hetuḥ prakṛtir ucyate puruṣaḥ sukhaduḥkhānāṃ bhoktṛtve hetur ucyate
Translation
Matter is called the cause of the generation of causes and effects; Spirit is called the cause of the enjoyment of pleasure and pain.
Annie Besant · The Bhagavad-Gita, translated by Annie Besant (4th ed., 1922) — Wikisource · Public Domain
Explanation
Verse 20 established that neither principle creates the other; this verse divides their functions instead. Prakriti is called the cause (hetu) in the production of effects and instruments — everything the field-catalogue of verses 6-7 listed — while purusha is called the cause specifically of experiencing pleasure and pain. The asymmetry is the whole argument: prakriti acts, purusha experiences, and mistaking the second role for the first is exactly the error the chapter is building toward correcting by verse 30.