Bhagavad Gita Chapter 2, Verse 17

अविनाशि तु तद्विद्धि येन सर्वमिदं ततम् | विनाशमव्ययस्यास्य न कश्चित्कर्तुमर्हति ||

avināśi tu tad viddhi yena sarvam idaṁ tatam vināśam avyayasyāsya na kaścit kartum arhati

Know that to be indestructible by which all this is pervaded. No one can destroy that immutable reality.

After distinguishing the real from unreal in verse 16, Krishna reveals what that 'sat' actually is: consciousness that pervades everything (tatam). Like space contains all objects yet can't be destroyed, consciousness holds all experience yet remains untouched. You can damage the eyes reading this, but can you damage the awareness itself? It has no parts to break. This is 'avyayasya'—the immutable reality. Your deepest identity isn't your vulnerable body or mind, but the indestructible awareness experiencing them.