An object is understood and known according to the expectation of the mind

Verse 4.17

तदुपरागापेक्षित्वात् चित्तस्य वस्तुज्ञाताज्ञातं ॥१७॥

tat-uparāga-apekṣitvāt-cittasya vastu jñāta-ajñātam ||17||

An object remains known or unknown according to the conditioning or expectation of the consciousness.

A conditioned mind can never perceive an object correctly. If the mind sees the object without expectation, it remains free.

An object is understood and known according to the expectation of the mind, or remains unrecognized owing to the absence of reflection. When the object attracts the mind, contact and reflection begin. 

This gives rise to knowledge. If the mind fails to come in contact with the object, it does not perceive it and the object remains unknown. If mind is conditioned or coloured knowledge of the object also becomes coloured. When the consciousness reflects on the object without condition, taint or expectation, its real essence is known. Similarly, if the mind reflects on the essence of the seer without conditioning, bias, or prejudice, the mind becomes enlightened.

It knows that it is not itself the seer, but only an instrument of the seer. The unenlightened mistake mind and consciousness for the seer.

PATANJALI YOGA SUTRAS