Yoga Sutra 3.35 knowledge of Purusa comes from what-is-for-the-sake-of-another

Sūtra III.35

Experience is an idea which does not distinguish between sattva and Puruṣa, though they are absolutely separate; by saṃyama on what-is-for-its-own-sake, (distinct) from what-is-for-the-sake-of-another, there comes knowledge of Puruṣa

Experience is an idea which does not distinguish between sattva and Puruṣa, though they are absolutely separate; by saṃyama on what-is-for-its-own-sake, (distinct) from what-is-for-the-sake-of-another, there comes knowledge of Puruṣa. Sattva is the mind (citta), Puruṣa is the experiencer (bhoktṛ); of these two which are absolutely separate, of absolutely opposed character and altogether separate existences, an idea which does not distinguish them, which takes them as the same, is experience of Puruṣa. By saṃyama on what-is-for-its-own-sake whose nature is pure consciousness, having distinguished it from what-is-for-the-sake-of-another, from sattva which by the idea of its being Puruṣa has the experience of being identical, comes knowledge of Puruṣa.

(Opponent) In what way is the mind-sattva for-the-sake-of-another, so that by making the distinction from it, saṃyama can be effected on what-is-for-its-own-sake?

(Answer) Mind-sattva (buddhi-sattva) always tends towards light –

Mind-sattva (buddhi-sattva) always tends towards light. Equally operative with sattva in the actualization of the purposes of Puruṣa, on the same basis and bound up with it, are the pair Rajas and Tamas. When they are mastered, when they have been subdued, mind-sattva is transformed by the yogin’s idea that sattva and Puruṣa are different.

That sattva is ever-changing because it consists of the three guṇa-s, because it is transient, because it is for-the-sake-of-another, because it is not conscious; absolutely opposite in character is Puruṣa, which is pure, different being unchanging, for-its-own-sake, eternal, and pure consciousness by nature.

Experience is an idea which does not distinguish between the two, although they are absolutely separate; (separate) because of the fact that Puruṣa has objects shown to it: this is the experience-idea. Experience cannot be the other, unchanging one, because that has objects shown to it. Sattva is for-the-sake-of-another, so experience, which is an idea that does not distinguish, is something seen by Puruṣa. Separate from that from that figurative idea and its possessor is the other idea of Puruṣa’s being pure consciousness by nature, the idea that Puruṣa alone is for-its-own-sake, since it has no change in it. From saṃyama on that is produced knowledge of Puruṣa.

(Opponent) But if Puruṣa is made the object of an idea, and Puruṣa is known by it, then Puruṣa ends up as for-the-sake-of-another.

(Answer) It is not that by any idea of Puruṣa, an idea with Puruṣa as object, which being an idea would be essentially mind-sattva, that being its quality, Puruṣa becomes seen. Because mind (buddhi) is not of itself conscious.

It is Puruṣa that sees the idea resting on his own self, a semblance of Puruṣa. The idea’s resting on Puruṣa is in fact resting on a semblance of Puruṣa; it is not that the idea does rest on Puruṣa. As a mirror set before a face is changed to the form of the face, so the mind (citta) changed into the form of Puruṣa, a semblance of Puruṣa, is what is perceived by Puruṣa.

And so it is said: By what, indeed, would one know the knower? (Bṛhad. Up. 4.5.15). The meaning is, that Puruṣa does not come to the state of an object for anyone.

(Opponent) just as the sight, striking the surface of the mirror and reflected back, is the perceiver of the face, so Puruṣa too, striking against the mind (citta) and reflected back, should perceive its own self.

(Answer) No, because Puruṣa has no changes, and being unchanging, it has no possibility of going forth towards the mind, or returning from it.

Nor, as a matter of fact, does sight bring about perception of the face by being reflected. For the face is experienced as located within the perimeter of the mirror. The face itself however is not located in the mirror.

Moreover, it conforms to the mirror. For instance, a face reflected in a sword is seen to be long, but the face itself is not long. If the perception were really by the sight reflected back, it would be illogical that it conforms to the mirror.