There is a tendency to think of oneself as a fixed thing, as unchanging. I am me and I am a certain way: I have a certain body, a certain emotional nature, certain interests, desires, and ways of thought. But this is in no way true.
Rather, that which I commonly think of as myself—the personality, including the body, habits, mental tendencies, and desires is constantly changing. Every thought that passes through the mind makes a small change in me.
If I act kindly that has an effect. If I act meanly it has an effect. Reading a book may influence me—and that creates a change. Even the body, which is the grossest and least malleable part of the personality, is constantly changing. If I compare a photograph of my body and face from ten years ago with my body now, I see much change.
There is one thing within me, however, that is not changing. According to the Yogis, it is always the same from birth to death, and even after death. It is not so much within me as it is me—the real me—the True Self.
The goal of Yoga is to find that one unchanging True Self.