Bhagavad Gita Chapter 13, Verse 32

यथा सर्वगतं सौक्ष्म्यादाकाशं नोपलिप्यते | सर्वत्रावस्थितो देहे तथात्मा नोपलिप्यते ||

yathā sarvagataṁ saukṣmyād ākāśaṁ nopalipyate sarvatrāvasthito dehe tathātmā nopalipyate

Just as the all-pervading space, because of its subtlety, is not tainted, so the Self, situated everywhere in the body, is not tainted.

Krishna uses the analogy of space to explain the Self's nature. 'Yathā sarvagataṁ saukṣmyād ākāśaṁ nopalipyate'—just as the all-pervading space, because of its subtlety, is not tainted. Space is everywhere, but it's not affected by what's in it. It doesn't get dirty, doesn't get attached, doesn't get tainted. 'Sarvatrāvasthito dehe tathātmā nopalipyate'—so also the Self, situated everywhere in the body, is not tainted. The Self is in the body, present everywhere in it, but it's not affected by the body. It doesn't get tainted by the body's actions, qualities, or changes. When you understand this, you recognize that you (the Self) are in the body, but you're not tainted by it. The body can be affected, but you remain pure. The body can change, but you remain constant. The body can be tainted, but you remain untainted.