Bhagavad Gita Verse 13, Chapter 2

dehinosminyathaa dehe kaumaram yauvanam jaraa |
tathaa dehaantarapraaptirdheerastatra na muhyati || 13 ||
 
Just as in this body, the body dweller passes through childhood, youth and old age, so also does it obtain another body (after death); the steadfast person does not grieve over this.
 
dehinaha : body dweller
asmin : this
yathaa : just like
dehe : body
kaumaram : childhood
yauvanam : youth
jaraa: old age
tathaa : similarly
dehaaantara : another
praaptihi: obtain
dheeraha : steadfast person
tatra : by that
na muhyati : do not grieve
 
It is our experience that we pass through childhood, youth and old age. We can agree that this concept is familiar to us. But note the language used in the first line. It is not you or I that passes through these phases, it is something called the "dehina" or the "body dweller". The body is born, it undergoes changes, and eventually perishes. But the body dweller remains constant through these changes. So, this means that the body dweller is something that is separate, distinct and different from the body.
 
Remember the example of Mr. X and his car from the first verse? Let's revisit it. Mr. X is excited when his car is brand new. After 5-6 years, it starts to develop engine problems. After another 4-5 years, the problems have become so bad that Mr. X decides to sell this car and buy a new Mercedes S-class. Mr. X can be called a "car dweller".
 
As the old car's engine degraded over the years, Mr. X remained the same from the car's perspective. But when the car had lived its life, he discarded that car for another new car. And there was nothing to be sad about this point. An extreme scenario is some unscrupulous people deliberately crash their old car just so that they can get insurance money to buy a new one.
 
Similarly, our body undergoes modifications of birth and aging, and eventually perishes. But the body dweller remains constant through these modifications. When the old body has become unfit to dwell in, the body dweller discards it and obtains a new body. The key point here is that the body dweller remains constant through the changes in its body, and also through the change from one body to another. And just like in the car example, a wise person should not grieve about growing old or dying, because the body dweller will always remain constant.
 
The body dweller is, therefore, the eternal essence that was highlighted in the prior verse. And since it is different than the physical body which perishes, it cannot be "found" in any part of the physical body.
 
So what exactly is this body dweller, this eternal essence? And how should we acquire the wisdom to see this eternal essence?
 
Footnotes
 
1. In one day we have several thoughts such as "I am happy", "I am sad", we join a condition to our "I". Each time do so, we are "born" as a happy person, as a sad person and so on, even if there is no new physical body that is born.