1.19. Just as a mirror exists within and without the image reflected in it, so the Supreme Self exists inside and outside this body.

The idea is this: The image in the mirror has no real existence. It is a mere appearance. Only the mirror exists. Similarly, only the Self exists. Body, mind, etc have no real existence. It is only by being superimposed on the Self that they appear to exist. Just as the reflection cannot affect the mirror, so body, mind etc cannot affect the Self.

aShTAvakra saMhitA, Swami Nityaswarupananda, Advaita Ashrama, No ISBN.