Kṣetra-Kṣetrajña Yoga · Verse 21
कार्यकारणकर्तृत्वे हेतुः प्रकृतिरुच्यते | पुरुषः सुखदुःखानां भोक्तृत्वे हेतुरुच्यते
kārya-kāraṇa-kartṛtve hetuḥ prakṛtir ucyate | puruṣaḥ sukha-duḥkhānāṃ bhoktṛtve hetur ucyate
In the production of effect and cause, nature is said to be the agent; the spirit is said to be the cause in the experience of pleasure and pain.
Prakṛti is the kārya-kāraṇa-kartrī — the doer of both effects (manifest world) and causes (subtle potential). All material activity occurs in nature.
Puruṣa is not an agent but a witness. Yet it is the hetu — cause — of experiencing (bhoktṛtva) because without consciousness, there would be no experience.
This is the Sāṅkhya doctrine of pariṇāma-vāda: transformation occurs in nature, not in spirit. Spirit provides the luminous ground on which nature’s transformations are reflected.