Kaṭha Upaniṣad · 2.2.10
वायुर्यथैको भुवनं प्रविष्टो रूपं रूपं प्रतिरूपो बभूव । एकस्तथा सर्वभूतान्तरात्मा रूपं रूपं प्रतिरूपो बहिश्च ॥ १० ॥
vāyuryathaiko bhuvanaṃ praviṣṭo rūpaṃ rūpaṃ pratirūpo babhūva | ekastathā sarvabhūtāntarātmā rūpaṃ rūpaṃ pratirūpo bahiśca || 10 ||
As wind, though it is one alone, upon entering the world assumes forms corresponding to each form, so the inner Ātman of all beings, being one, assumes forms corresponding to each form and is beyond all forms.
The vāyu (wind) is an even subtler symbol than fire for representing the Ātman. While fire needs fuel to manifest, wind can move freely through all spaces without being contained by any. It is invisible in itself but perceivable by its effects.
This metaphor illustrates how the Ātman penetrates all bodies as prāṇa (vital energy) without being limited by any. The wind that circulates through different rooms does not lose its nature as wind; thus, the Ātman passes through all forms of existence while maintaining its essential unity.
In yoga, this teaching liberates us from identification with the particular body. If we are the wind that animates the form, not the form itself, then we are not bound by the limitations of our physical body. The practice of prāṇa awareness helps us directly experience this truth of our nature as universal vital energy.