Maitri (Maitrāyaṇīya) Upaniṣad 2.5

2.5

sa vā eṣa sūkṣmo'grāhyo'dṛśyaḥ puruṣasaṃjñako buddhipūrvamihaivāvartateṃ'śena suṣuptasyaiva buddhipūrvaṃ nibodhayatyatha yoha khalu vāvāitasyāṃśo'yaṃ yaścetanamātraḥ pratipūruṣaṃ kṣetrajñaḥ saṅkalpādhyavasāyābhimānaliṅgaḥ prajāpatirviśvakṣastena cetanenedaṃ śarīraṃ cetanavatpratiṣṭhāpitaṃ pracodayitā caiṣo'syeti te hocurbhagavannīdṛśasya kathamaṃśena vartanamiti tānhovāca ||

5. “Verily, that subtile, ungraspable, invisible one, called the Person, turns in here [in the body] with a part [of himself] without there being any previous awareness, even as the awakening of a sleeper takes place without there being any previous awareness.

Now, assuredly, indeed, that part of Him is what the intelligence-mass here in every person is—the spirit (_kṣetrajña_, ‘knower-of-the-body’) which has the marks of conception, determination, and self-conceit (_abhimāna_), Prajāpati (Lord of Creation) under the name of individuality.

By Him, as intelligence, this body is set up in possession of intelligence; or, in other words, this very one is its driver.”

Then they said: “Sir, if by this kind of indifferent being this kind of body is set up in possession of intelligence, still how, in other words, is this one its driver?”

Then he said to them:—