Patanjali's yoga sutras

सत्त्वपुरुषयोर् अत्यन्तासंकीर्णयोः प्रत्ययाविशेषो भोगः परार्थात् स्वार्थसंयमात् पुरुषज्ञानम्

sattvapuruṣayor atyantāsaṃkīrṇayoḥ pratyayāviśeṣo bhogaḥ parārthāt svārthasaṃyamāt puruṣajñānam

sattva—of the objective essence

puruṣayoḥ—and of the Puruṣa

atyanta—quite

asaṅkīrṇayoḥ—distinct from each other

pratyaya—the notion of distinction

aviśeṣaḥ—the absence

bhogaḥ—is experience

parārthatvāt—because it exists for another

svārtha—on his own object

saṃyamāt—by Saṃyama

puruṣa-jñānam—the knowledge of the Puruṣa.

Even in its purest form, the field of matter and energy to which your mental vortexes belong is part of existence, what is known, making it quite distinct from Consciousness, the knower. What is known and the knower are in opposite categories. Consciousness, the Self, is never an object of experience; it is the subject, the one who is aware, the one who knows. Existence comes into being to be known by Consciousness. Consciousness comes into being to know Existence. By training your focus on your own object, your own mind, you gain the insight of Consciousness.

Vyāsa, in the Sāṅkhya–pravacana, his 7th century commentary on Patanjali’s Yoga Sutras, connects this sutra to the Brihadaranyaka Upanishad.

https://www.wisdomlib.org/hinduism/book/yoga-sutras-with-commentaries/d/doc1216741.html

Br. Up. II. 4-14):—‘And so it has been said’ by Īśvara:—‘By what Oh! is the knower to be known?’ That is, by nothing.

https://www.wisdomlib.org/hinduism/book/the-brihadaranyaka-upanishad/d/doc117950.html

Verse 2.4.14:

यत्र हि द्वैतमिव भवति तदितर इतरं जिघ्रति, तदितर इतरं पश्यति, तदितर इतरम् श्र्णोति, तदितर इतरमभिवदति, तदितर इतरम् मनुते, तदितर इतरं विजानाति; यत्र वा अस्य सर्वमात्माइवाभूत्तत्केन कं जिघ्रेत्, तत्केन कं पश्येत्, तत्केन कं शृणुयत्, तत्केन कमभिवदेत्, तत्केन कं मन्वीत, तत्केन कं विजानीयात्? येनेदम् सर्वं विजानाति, तं केन विजानीयात्? विज्ञातारम् अरे केन विजानीयादिति ॥ १४ ॥ 
इति चतुर्थं ब्राह्मणम् ॥

yatra hi dvaitamiva bhavati taditara itaraṃ jighrati, taditara itaraṃ paśyati, taditara itaram śrṇoti, taditara itaramabhivadati, taditara itaram manute, taditara itaraṃ vijānāti; yatra vā asya sarvamātmāivābhūttatkena kaṃ jighret, tatkena kaṃ paśyet, tatkena kaṃ śṛṇuyat, tatkena kamabhivadet, tatkena kaṃ manvīta, tatkena kaṃ vijānīyāt? yenedam sarvaṃ vijānāti, taṃ kena vijānīyāt? vijñātāram are kena vijānīyāditi || 14 || 
iti caturthaṃ brāhmaṇam ||

14. Because when there is duality, as it were, then one smells something, one sees something, one hears something, one speaks something, one thinks something, one knows something. (But) when to the knower of Brahman everything has become the Self, then what should one smell and through what, what should one see and through what, what should one hear and through what, what should one speak and through what, what should one think and through what, what should one know and through what? Through what should one know That owing to which all this is known—through what, O Maitreyī, should one know the Knower ?

Even in the state of ignorance, when one sees something, through what instrument should one know That owing to which all this is known? For that instrument of knowledge itself falls under the category of objects. The knower may desire to know not about itself, but about objects. As fire does not burn itself, so the self does not know itself, and the knower can have no knowledge of a thing that is not its object. Therefore through what instrument should one know the knower owing to which this universe is known, and who else should know it? And when to the knower of Brahman who has discriminated the Real from the unreal there remains only the subject, absolute and one without a second, through what instrument, O Maitreyī, should one knowthat Knower?

You Are It:
The Knower That Cannot Be Known,
The Source of All Knowing

“So Shankara gets at it when he says,
“That which is the knower or the knowing in everything can never itself be an object of its own knowledge; for fire doesn’t burn itself,” although it burns other things. So we never know what the Brahman is, just like the eyes don’t ever see the head. If you put something there, you are stopping short of nothing and you don’t get the whole benefit of it, that’s all…
Now, I really think that’s the simplest thing I can possibly tell you. I really don’t know what else there is to be said about this whole Zen project, or mysticism, Vedānta, what have you. It comes down to that, and there are infinitely many ways of evading.”

— Alan Watts, ‘The Inevitable Ecstasy (Part 2), Out of Your Mind 6’, at 01:14:17

Shankara explains it beautifully in his commentary on the Kena Upanishad, where he says that that which is the knower—the ground of all knowledge—is never itself an object of knowledge, just as fire doesn’t burn itself.

https://www.organism.earth/library/document/tao-of-philosophy-7