Bhagavad Gita, Chapter 13, Verse 33

As the all-pervading Akâsha, because of its subtlety, is not tainted, so the Self existent in the body everywhere is not tainted.

A real, familiar image, space holding everything without being changed by any of it, describes something otherwise hard to picture.

Comparing the soul to space gives real, physical intuition to an abstract claim, presence without contamination. The comparison holds because both are shown as genuinely untouched by what surrounds them.