In plain words
Prakṛti is said to be the cause in the producing of body and organs; Puruṣa is said to be the cause of experiencing pleasure and pain.
Word for word
In the producing of body and organs prakṛti is said [to be] the cause; puruṣa is said [to be] the cause in the experiencing of pleasures and pains.
कार्यकरणकर्तृत्वे हेतुः प्रकृतिरुच्यतेपुरुषः सुखदुःखानां भोक्तृत्वे हेतुरुच्यते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
के पुनः ते विकाराः गुणाश्च प्रकृतिसम्भवाः —
कार्यकरणकर्तृत्वे — कार्यं शरीरं करणानि तत्स्थानि त्रयोदश । देहस्यारम्भकाणि भूतानि पञ्च विषयाश्च प्रकृतिसम्भवाः विकाराः पूर्वोक्ताः इह कार्यग्रहणेन गृह्यन्ते । गुणाश्च प्रकृतिसम्भवाः सुखदुःखमोहात्मकाः करणाश्रयत्वात् करणग्रहणेन गृह्यन्ते । तेषां कार्यकरणानां कर्तृत्वम् उत्पादकत्वं यत् तत् कार्यकरणकर्तृत्वं तस्मिन् कार्यकरणकर्तृत्वे हेतुः कारणम् आरम्भकत्वेन प्रकृतिः उच्यते । एवं कार्यकरणकर्तृत्वेन संसारस्य कारणं प्रकृतिः । कार्यकारणकर्तृत्वे इत्यस्मिन्नपि पाठे, कार्यं यत् यस्य परिणामः तत् तस्य कार्यं विकारः विकारि कारणं तयोः विकारविकारिणोः कार्यकारणयोः कर्तृत्वे इति । अथवा, षोडश विकाराः कार्यं सप्त प्रकृतिविकृतयः कारणम् तान्येव कार्यकारणान्युच्यन्ते तेषां कर्तृत्वे हेतुः प्रकृतिः उच्यते, आरम्भकत्वेनैव । पुरुषश्च संसारस्य कारणं यथा स्यात् तत् उच्यते — पुरुषः जीवः क्षेत्रज्ञः भोक्ता इति पर्यायः, सुखदुःखानां भोग्यानां भोक्तृत्वे उपलब्धृत्वे हेतुः उच्यते ॥
कथं पुनः अनेन कार्यकरणकर्तृत्वेन सुखदुःखभोक्तृत्वेन च प्रकृतिपुरुषयोः संसारकारणत्वमुच्यते इति, अत्र उच्यते — कार्यकरणसुखदुःखरूपेण हेतुफलात्मना प्रकृतेः परिणामाभावे, पुरुषस्य च चेतनस्य असति तदुपलब्धृत्वे, कुतः संसारः स्यात् ? यदा पुनः कार्यकरणसुखदुःखस्वरूपेण हेतुफलात्मना परिणतया प्रकृत्या भोग्यया पुरुषस्य तद्विपरीतस्य भोक्तृत्वेन अविद्यारूपः संयोगः स्यात् , तदा संसारः स्यात् इति । अतः यत् प्रकृतिपुरुषयोः कार्यकरणकर्तृत्वेन सुखदुःखभोक्तृत्वेन च संसारकारणत्वमुक्तम् , तत् युक्तम् । कः पुनः अयं संसारो नाम ? सुखदुःखसम्भोगः संसारः । पुरुषस्य च सुखदुःखानां सम्भोक्तृत्वं संसारित्वमिति ॥ २० ॥
In plain words
But which ones are they — those modifications, and the guṇas, that come out of prakṛti?
On the matter of agency over effect and instrument: the 'effect' is the body; the 'instruments' are the thirteen faculties that reside within it. The five elements that build the body, and the sense-objects — both of them modifications arising from prakṛti, already discussed earlier — are here gathered under the single word 'effect.' And the guṇas — pleasure, pain, and delusion, also born from prakṛti — since they underlie the instruments, are gathered under the word 'instrument.' Their agency, their capacity to produce anything at all, is what 'agency over effect and instrument' means; and the cause behind that agency, acting as the initiator, is called prakṛti. So it is prakṛti, through its agency over effect and instrument, that is the cause of the whole cycle of birth and death. Under a slightly different reading of the text, the same idea works a little differently: whatever is a transformation of something else is called its 'effect' or 'modification,' and whatever undergoes that transformation is called its 'cause' — and it is the agency over this pair, modification and modifiable, that is being discussed here. Or, taken yet another way: the sixteen evolutes count as 'effect,' and the seven primary transformations of prakṛti count as 'cause' — together they are simply called effect and cause. And the reason behind their agency, again, is called prakṛti — but only as their initiator, nothing more. Now, how does puruṣa become the cause of saṃsāra? Here is the answer: puruṣa, the individual soul, the field-knower, the enjoyer — these are all just different names for the same thing. And puruṣa is said to be the cause of enjoying and perceiving pleasure and pain, the very things there to be enjoyed.
So how exactly, someone might object, can saṃsāra be caused jointly by prakṛti and puruṣa — through prakṛti's doing and through puruṣa's experiencing pleasure and pain? Here is the answer. Suppose prakṛti never actually changed into the shape of causes and effects, pleasures and pains — and suppose the conscious puruṣa never actually took any of that in. Then where would saṃsāra even come from? It couldn't. But here is what actually happens: prakṛti does transform into that fourfold shape and becomes the thing to be enjoyed, and puruṣa, standing opposite it, takes on the role of enjoyer. Between them a connection forms — and that connection is nothing but ignorance wearing the mask of intimacy. Once that happens, saṃsāra is exactly what results. So the old claim holds after all: prakṛti and puruṣa really are joint causes of saṃsāra, through doing on one side and experiencing on the other. But what is saṃsāra, once you strip the jargon away? Simply this — undergoing pleasure and pain. And puruṣa's being caught up in undergoing them, nothing more mysterious than that, is what it means to be bound to saṃsāra.
Close to the Sanskrit
But which are those modifications, and the guṇas, arising from prakṛti —
With regard to agency over effect and instrument — : effect [is] the body; instruments [are] the thirteen situated in that [body]. The five elements, the originators of the body, and the objects — [being] modifications born from prakṛti, already mentioned — here, are included by the term 'effect'. And the guṇas, born from prakṛti, consisting of pleasure, pain, and delusion, because they are the basis of the instruments, are included by the term 'instrument'. Their — [that is] the effect's and instrument's — agency, [that is] the state of producing — that which is — [is called] 'agency over effect and instrument'; in that agency over effect and instrument, the cause, [that is] the reason, by way of being the originator, is called prakṛti. Thus, by [being the cause of] agency over effect and instrument, prakṛti [is] the cause of transmigration. In this reading also — [namely] 'kāryakāraṇakartṛtve' — [here] effect [is] that which is a transformation of something; that [is] its effect, [that is] modification; [and] the modifiable [is the] cause — of those two, of the modification and of that-which-is-modified, [that is] of effect and cause, in [their] agency — thus [this reading is explained]. Or alternatively, the sixteen modifications [are] the effect, [and] the seven transformations of prakṛti [are] the cause — those very [sixteen and seven] are called effect and cause; of their agency, the cause, [that is] the reason, is called prakṛti, by way of being the originator alone. And how puruṣa would be the cause of transmigration — that is [now] stated — puruṣa, jīva, kṣetrajña, bhoktā [the enjoyer] — these are synonyms; [puruṣa] is said to be the cause of the enjoyership and perceivership of pleasures and pains, [which are] to be enjoyed.
How again, by this — by agency in effect-and-instrument, and by experiencership of pleasure-and-pain — is the causehood of saṃsāra of prakṛti and puruṣa said to hold? Here it is said: in the absence of a transformation of prakṛti in the nature of cause and effect, having the shape of effect, instrument, pleasure and pain — and puruṣa, who is conscious, not being the apprehender of that — from where would saṃsāra be? But when, prakṛti having transformed into the nature of cause and effect, in the shape of effect, instrument, pleasure and pain, being the thing enjoyed, there would be, through puruṣa's experiencership, he being opposite to that, a conjunction in the nature of ignorance — then saṃsāra would be. Therefore what was said — that the causehood of saṃsāra of prakṛti and puruṣa is by agency in effect-and-instrument and experiencership of pleasure-and-pain — that is correct. What again is this thing called saṃsāra? The undergoing of pleasure and pain is saṃsāra; and puruṣa's being the undergoer of pleasures and pains is the state of being subject to saṃsāra.