Excerpts from books, lectures, and videos by Jagad Guru Chris Butler (Siddhaswarupananda Paramahamsa).
If you are not the gross physical body and you are not the mind, then who are you? What is your essence?
According to yoga, you are made of an energy completely distinct from matter. You, the self (atma), are an indivisible unit of the element known as life.
Thus, there are two fundamental energies—matter and life. Your body is made of the element matter, but you, the atma, are a spark of the element life. When you, the life particle, are in or wedded to a material body, that body is called a living organism. So the answer to the question "Who/what are you?" is: You, the self (atma), are a particle of the element life. You are presently within a material body. You are temporarily possessing and using that body.
According to the science of yoga, you, the life particle, are seated in the heart (but not inseparably—you can remove the physical heart, but you will not be removed with it). Your influence spreads throughout the body by means of this vast network of channels (nadis) that carry the life force to every cell of the body.
It is important to know that you are not material in essence but rather an eternal spark of life. But such knowledge is just the beginning of a long and arduous journey toward a perfect understanding of your identity.