Chapter 1, Verse 2 of the Yoga Sutras: Yogas citta vrtti nirodha. Translation: Yoga is the stilling of the fluctuations of the mind.
Four Sanskrit words, with all of the meaning of yoga contained therein (they aren't called "Sutras" for nothing; sutra means "thread" - think "suture" - in this case the barest thread of a narrative, completely unadorned by interpretation, which is left entirely to the reader/student).... Yoga is the stilling of mental activity. The next sutra tells us WHY we wish to quiet the chatter running through our minds: Only then do we abide in our "true nature".
Our "true nature" can be viewed as who each of us is underneath all of the layers of experience, knowledge, memories, even dreams that pile upon us as we live our lives. To those who believe in God, our "true nature" can be viewed as the part within us that is Godlike, divine, untouched by the subjective or the relative. Thus, by definition, "our true nature", also known as "the self", is the same in each of us. Underneath all of the layers of experience, knowledge, memories and dreams, each of us is essentially the same.
The poet, Wallace Stevens, often wrote of the transformation of "reality" into "something else" simply by virtue of the mind's perceiving that reality. In "The Man with the Blue Guitar," he wrote:
They said, "You have a blue guitar,If somehow we could manage to quiet the guitar, perhaps we could play a tune not "beyond ourselves" but OF ourselves.
You do not play things as they are."
The man replied, "Things as they are
Are changed upon the blue guitar."
And they said then, "But play, you must,
A tune beyond us, yet ourselves,
A tune upon the blue guitar
Of things exactly as they are."
One could think of a newborn baby in his first nanosecond of life as the "pure self" of the person he will be (although that would not address the possibility of the experiences, knowledge, memories and dreams of that person's prior lives). Each moment of his life, even before he possesses the power of language, he has experiences. Those experiences create waves of thought in his mind. Those waves of thought color future experiences. The blue guitar is already playing. The latent impressions, assumptions and tendencies that comprise the "blue guitar" are known in yoga philosophy as "samaskaras". Easy to remember, because it sounds so much like "some scars of ours".
The state of "yoga" is the quieting of that blue guitar. It is not the process of creating a "numb" mind or eliminating all of our acquired knowledge. Rather, it is the process of untying ourselves from the blue guitar, of not identifying with our thoughts, our emotions, our experiences, of not confusing those things with "reality", which is a constant. Thoughts, emotions, experiences all are transient. They come and they go and they come again and then there they go....poof. Through the practice of yoga, we can come to break our identification with that which is external, recognizing it as external, entirely outside of our control and entirely separate from the our true nature, the "self".
In thinking about what is "the self" versus what is "the mind" it might be helpful to consider this: When you talk to yourself, who is listening?