कार्यकारणकर्तृत्वे हेतुः प्रकृतिरुच्यते।
पुरुषः सुखदुःखानां भोक्तृत्वे हेतुरुच्यते।।13.21।।
13.21 With regard to the source of body and organs, Nature is said to be the cause. The soul is the cause so far as enjoyership of happiness and sorrow is concerned.
13.21 The Karya means the body, the Karanas mean the instruments, i.e., the senses of perception and action plus the Manas. In their operations, the Prakrti, subservient to the self, is alone the causal factor. The sense is that their operations, which are the means of experience, have their foundation in the Prakrti, which has developed in the form of the body subservient to the self. In regard to this, the authority is the aphorism, The self is an agent, on account of the scriptures having the purpose (B. S., 2.3.33) etc. The agency of the self means that the self is the cause of the will (effort) to support the body. The self (Purusa) associated with the body is the cause for experiencing pleasures and pains. The meaning is that It is the seat of those experiences. Thus, has been explained the difference in the operations of the Prakrti and of the self when they are mutually conjoined. He now proceeds to explain how, though the self, which in Its pristine nature experiences Itself by Itself as nothing but joy, becomes the cause of experiencing both pleasure and pain derived from sense objects when It is conjoined with a body. The term Guna figuratively represents effects. The self (in Its pristine nature) experiences Itself by Itself, as nothing buy joy. But when dwelling in the body, i.e., when It is in conjunction with the Prakrti, It experiences the alities born of Prakrti, namely, happiness, pain etc., which are the effects of Gunas like Sattva etc. He explains the cause of conjunction with the Prakrti:
Kaaryakaaranakartrutwe hetuh prakritiruchyate; Purushah sukhaduhkhaanaam bhoktritwe heturuchyate.
kārya—effect; kāraṇa—cause; kartṛitve—in the matter of creation; hetuḥ—the medium; prakṛitiḥ—the material energy; uchyate—is said to be; puruṣhaḥ—the individual soul; sukha-duḥkhānām—of happiness and distress; bhoktṛitve—in experiencing; hetuḥ—is responsible; uchyate—is said to be