Bhagavad Gita Chapter 13 Verse 21
भगवद् गीता अध्याय 13 श्लोक 21

कार्यकारणकर्तृत्वे हेतुः प्रकृतिरुच्यते।
पुरुषः सुखदुःखानां भोक्तृत्वे हेतुरुच्यते।।13.21।।



English Translation - Swami Sivananda
13.21 In the production of the effect and the cause, Nature (matter) is said to be the cause; in the experience of pleasure and pain, the soul is said to be the cause.


English Translation of Sanskrit Commentary By Sri Shankaracharya's
13.21 Karya-karana-kartrtve, with regard to the source of body and organs: Karya is the body, and karana are the thirteen [Five sense organs, five motor organs, mind, intellect and ego.] organs existing in it. Here, by the word karya are understood the aforesaid elements that produce the body as also the objects which are modifications born of Nature. And since the alities-which are born of Nature and manifest themselves as happiness, sorrow and delusion-are dependent on the organs, (therefore) they are implied by the word karana, organs. The kartrtvam, (lit) agentship, with regard to these body and organs consists in being the source of the body and organs. With regard to this source of the body and organs, prakrtih, Nature; ucyate, is said to be; the hetuh, cause, in the sense of being the originator. Thus, by virtue of being the source of body and organs, Nature is the cause of mundane existence.
Even if the reading be karya-karana-kartrtva, karya (effect, modification) will mean anything that is the transformation of something; and karana (cause) will be that which becomes transformed. So the meaning of the compund will be: with regard to the source of the effect and the cause.
Or, karya means the sixteen [The eleven organs (five sensory, five motor, and mind) and the five objects (sound etc.).] modificaitons, and karana means the seven [Mahat, egoism, and the five subtle elements.] transformations of Nature. They themselves are called effect and cuase. So far as the agentship with regard to these is concerned Nature is said to be the cause, because of the same reason of being their originator.
As to how the soul can be the cause of mundane existence is being stated: Purusah, the soul, the empirical being, the knower of the field-all these are synonymous; is the hetuh, cause; bhoktrtve, so far as enjoyership, the fact of being the perceiver; sukha-duhkhanam, of happiness and sorrow-which are objects of experience, is concerned.
How, again, is it asserted with respect to Nature and soul that, they are the causes of mundane existence by virtue of this fact of their (respectively) being the source of body and organs, and the perceiver of happiness and sorrow?
As to this the answer is being stated: How can there be any mundane existence if there be no modification of Nature in the form of body and organs, happiness and sorrow, and cause and effect, and there be no soul, the conscious being, to experience them? On the other hand, there can be mundane existence when there is a contact, in the form of ignorance, between Nature-modified in the form of body and organs, and cause and effect as an object of experience and the soul opposed to it as the experiencer. Therefore it was reasonable to have said that, Nature and soul become the cause of mundane existence by (respectively) becoming the originators of the body and organs, and the perceiver of happiness and sorrow.
What again is this that is called worldly existence? Worldly existence consists in the experience of happiness and sorrow; and the state of mundane existence of the soul consists in its being the experiencer of happiness and sorrow.
It has been asserted that the state of mundane existence of the soul consists in its being the experiencer of happiness and sorrow. How does it come about? This is being answered:


Transliteration Bhagavad Gita 13.21Kaaryakaaranakartrutwe hetuh prakritiruchyate;
Purushah sukhaduhkhaanaam bhoktritwe heturuchyate.


Word Meanings Bhagavad Gita 13.21kārya—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