अग्निर्यथैको भुवनं प्रविष्टो रूपं रूपं प्रतिरूपो बभूव ।
एकस्तथा सर्वभूतान्तरात्मा रूपं रूपं प्रतिरूपो बहिश्च ॥ ९ ॥

agniryathaiko bhuvanaṃ praviṣṭo rūpaṃ rūpaṃ pratirūpo babhūva |
ekastathā sarvabhūtāntarātmā rūpaṃ rūpaṃ pratirūpo bahiśca || 9 ||

9. As fire, though one, having entered the world, takes a separate form in respect of every form, so does the internal atman of all living things assume a form for every form and is outside all forms.

Shankara’s Commentary:

As the knowledge of the oneness of the atman, though supported by authority and often reiterated, is not firmly grasped by the intellect of perverse-headed Brahmins, whose mind is shaken by the intellect (arguments) of many logicians, the sruti, being anxious to inculcate it, says again and again: as fire, though one, bright by nature, having entered the world (Bhuvanam meaning world, because all bhavanti, i.e., are born there) in respect of everything to be burnt assumes separate shapes; so, being one only, the internal atrnan of all, having entered all bodies such as fire in logs, etc., being extremely subtle, assumes forms responsive and is also without them all, like the akas, in his own unmodified nature.