Just as space, which is present everywhere, is not stained because of its subtlety, so the self, present everywhere in the body, is not stained.
Explanation
Krishna continues explaining why the self (ātman) is untouched even while living in the body, as the previous verse said it neither acts nor is tainted. His image is space (ākāśa): it pervades everything, yet it is so subtle that mud, smoke or anything else cannot cling to it. In the same way, the self is present throughout the body without being affected by its pleasures, pains or actions.