Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.7.17

4.6.7.17

tairupāṃśvacaran | āpyāyayannevaināni tattata eṣā vidyetare vidye pratyāsīttato 'ntarikṣaloka itarau lokau pratyāsīttasmādyajūṃṣi niruktāni santyaniruktāni tasmādayamantarikṣaloko niruktaḥ sannaniruktaḥ

They performed with them in a low voice, and thereby strengthened them; and henceforth that science was equal to the other two sciences, and the air-world was equal to the two other worlds. Therefore the Yagus, while being distinct, are yet indistinct; and therefore the air-world, while being distinct, is yet indistinct (indefinable).