The brief comparison with Buddhist anatta in the main entry leaves out the rich textual development of atman within the Upanishads, which supply several distinct and durable images for what the doctrine claims.
The Katha Upanishad's chariot analogy is among the most influential: the body is the chariot, the intellect (buddhi) is the charioteer, the mind (manas) is the reins, the senses are the horses, and the sense objects are the roads they travel, while the atman is the passenger, present throughout the journey but distinct from every part of the vehicle, an image organizing the self's relation to body, mind and senses hierarchically rather than merely asserting a self exists. The Chandogya Upanishad's teaching of Uddalaka to his son Svetaketu supplies the tradition's most quoted identity statement, tat tvam asi, "you are that," repeated after several illustrations, salt dissolved in water remaining present though invisible, a fig seed containing a tree in potential, arguing the same subtle essence pervading the universe is identical with the individual self, a text later Advaita commentators treat as one of the four canonical "great sayings" (mahavakya).
These images work by different routes toward the same claim the main entry states abstractly, that beneath the changing contents of experience stands an unchanging witness, and their durability reflects their concreteness: rather than arguing philosophically that a permanent self must exist, they offer a picture that lets the doctrine be grasped before it is examined.