Purusha is ever illuminative and changeless

Verse 4.18

सदाज्ञाताः चित्तव्र्त्तयः तत्प्रभोः पुरुषस्यापरिणामित्वात् ॥१८॥

sadā jñātāḥ citta-vṛttayaḥ tat-prabhoḥ puruṣasya-apariṇāmitvāt ||18||

Purusha is ever illuminative and changeless. Being constant and master of the mind, he always knows the moods and modes of consciousness.

The Lord of mind is the seer. He is changeless, constant, and never alters or falters.

In deep sleep, mind forgets itself. It is purusha, as a witness which reminds the mind of sleep after waking. 

This indicates that purusha is ever alert and aware. Purush's alertness will be known to the sadhaka only when consciousness is purified and freed from rising thoughts and their restraints. Then the sadhaka, the seeker, becomes the seer.

The seer knows his consciousness and its ramifications. He is the seed

and root, and consciousness is the seedling. Its stem is the 'I' Consciousness , which branches forth as ego, intelligence and mind. The seer, being the seed and root of consciousness, observes the changes and transformations

taking place in it.

PATANJALI YOGA SUTRAS