The eighteenth yoga sutra explains about the immense significance of purusa, which is eternally alert and illuminative. It is the only invariable and master factor of the mind and is knowledgable about the mind`s every move and step. However, a normal human never recognises purusa`s presence. It is only when he has become an enlightened yogi free from all limitations, that he comes to know about its nature. Then a sadhaka- the seeker, turns a seer.

Sada always
Jnatah known
citta vrttayah fluctuations of the consciousness
tatprabhoh of its lord
purusasya of the soul
aparinamitvat an account of changelessness

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

The Lord of consciousness is the seer. He is changeless, constant, and never alters or stumbles.

In deep sleep, consciousness forgets itself. It is purusa, as a witness which reminds the mind of sleep after waking. This designates that purusa is ever alert and aware (sada jnata). Purusa`s alertness will be known to the sadhaka only when consciousness is purified and liberated from rising thoughts and their restraints. Then the sadhaka, the seeker, becomes the seer.

The seer knows his consciousness and its branchings. He is the seed and root, and consciousness is the seedling. Its stem is the `T` consciousness (asmita), which branches forth as ego, intelligence and mind. The seer, being the seed and root of consciousness, observes the changes and transformations taking place within it. (II.17, 20, 22-24; iv.30.)