आत्मा नित्यो हि सद्रूपो देहोऽनित्यो ह्यसन्मयः ।तयोरैक्यं प्रपश्यन्ति किमज्ञानमतः परम् ॥ २१ ॥
In plain words
The Ātman is eternal; Being itself is its nature. The body is impermanent, made through and through of what is not. And people look at these two and see one thing. What ignorance could go further than that?
Close to the Sanskrit
The Self is indeed eternal, having Being as its nature; the body is impermanent, indeed made of non-being. Of those two they see identity — what ignorance is greater than this?