I believe, from my own contemplation and studies over the years, it is the portion of our psyche shaped by and focused on the illusions of the physical world. These illusions are passed to us from a collective preconditioned mode of thought that we acquire in infancy.
The search for "The Self" is manifold and reaches past the physical brain. A magnificent organ, wired to respond and process stimuli.
The Self is not the human body. Humans are not just a physical body wired wondrously to the organs and systems within.
The physical body is a finely constructed vehicle to aid The Self while navigating through the physical world. The Self is both connected to the physical body and it is remote.
Idealized restrictions of thought shaped by our preconditioning have confined the totality of humans to a physical body.
We are more than that.
Humankind has manifested all manner of things, even with these self imposed restrictions.
Developing systems of reaching past the confines of precondtioned and reflexive thought patterns.
Through religion and systems of ritualized seeking into the unknown and unseen. Delving into the mysteries of life through seemingly hidden compartments of the human body and modes of stylized thought patterns.
These are not hidden compartments. In truth, they are but obscured and forgotten pathways. Once found they become more accessible and far reaching.
And then we discover, we are more than our physical bodies. The common verb becomes the Ultimate Noun.