Verse 21

एकस्मिन्नेव सततं स्थिते सन्मात्रवस्तुनि ।जातोऽयमयमुन्नष्ट इति तेषां तवेह धीः

In plain words

While the one reality, mere being, stands ever constant -- 'this one is born, this one has perished': such is your understanding of them, here.