kaaryakaranakartritve hetuhu prakritiruchyate |
purushaha sukhaduhkhaanaam bhoktritve heturuchyate || 20 ||

With regard to the source of actions in the body and its instruments, Prakriti is said to be the cause. With regard to experiencing joy and sorrow, Purusha is said to be the cause.
kaarya : body
karana : instruments
kartritve : source of actions
hetuhu : cause
prakriti : Prakriti
uchyate : it is said
purushaha : Purusha
sukhaduhkhaanaam : joy and sorrow
bhoktritve : experience
hetuhu : cause
uchyate : it is said
Let us bring back the example of the movie projector from the seventh chapter so that we can better understand this shloka. Imagine a gigantic IMAX screen on which Prakriti or nature projects the life of every human being in the world. Think of it as the world’s largest soap opera. We can now examine the first half of this shloka. It shows what Prakriti can create through its projections. It can project “kaarya”, the body of a person. It can project “karana”, the instruments of the body which include the five organs of sense, five organs of actions, the mind, the intellect and the ego. It can not just project one body, but every body in the world.
Now, here comes an important point. Shri Krishna says that Prakriti is the source of all actions in this world, not the Purusha. We are now coming back to the topic that was hinted upon in the karma yoga chapter. Typically, most of us attribute the agency, or the doership of our actions, to our own self. We say “I did this”, “I did not do that” and so on. Shri Krishna makes it perfectly clear that the intellect, the ego and the mind in our body receive input from our senses, filter it through our vaasanaas, and send instructions to our organs of action. All this is going on within the realm of Prakriti, that continues projecting the IMAX movie of the world. In other words, the “I” does not do anything, but Prakriti does everything.
Next, the role of the Purusha is described. The Purusha is the awareness principle, the knowledge principle present in the body. From our perspective, he is concerned with the experience of only one body out of the millions of bodies in that IMAX movie – our body. What is his role? His role is to know. If we put a drop of a bitter liquid on our tongue, it sends an electrical current to the mind based on the chemical makeup of the bitter liquid. But ultimately, it is only the Purusha that has the capacity to come up with the knowledge that “this liquid is bitter”. On one level, Purusha knows what the senses and the mind report. Without the Purusha, there will be nothing to know what Prakriti has projected. It would be like projecting a movie without an audience to see it.
So then, when this knowledge of bitterness is filtered through our vaasanaas or our “programming”, it can result in either joy or sorrow. Some of us like bitter taste, some of us don’t. This difference comes from the variety in our vaasanaas, our individual programming. So whenever external objects are arranged by Prakriti in a pattern that is conducive to our vaasanaas, the Purusha experiences joy. In other words, whenever we say “I am happy”, it is the Purusha experiencing happiness. Similarly, sorrow is also experienced when objects are undesirable.
Here, encapsulated in these two lines of this shloka, is the state of our lives. Our body with its organs interacts with other bodies in this world. It performs actions whose results are experienced by the Purusha as joy and sorrow. The cycle of joy and sorrow continues from one action to another action, from one experience to another experience. This is “samsaara”.
Now, there seems to be a problem. Right from the second chapter, we have been told that our true nature is the eternal essence. It pervades the entire universe. It is eternal, indestructible and indivisible. We have also been told that Prakriti, through some inexplicable magic, projects the entire universe of names and forms. How then, does the third entity called Purusha come into being? And also, how does it take on one body out of all the bodies in the world as its own, and experience only that body’s joy and sorrow?
Shri Krishna reveals the root cause of samsaara, of our repeated experience of joy and sorrow, in the next shloka.