iv. 18-22 The Self is the Master over all thought-constructions.
4.18 The changeless Selfhood of spirit is Master over the churning of incessant thoughts.
Thought being constructed out of the phenomenal mind-stuff is inferior to the Self that is Self-Luminous and all-knowing. The thought-realm is heavy laden with obstructive concerns and is not even able to know the true-nature of itself as knowable objects. The Self alone sheds the Illuminative Light of gnosis over all matter. Hence the Purusha (Self) is the Supreme Lord over all phenomenlizations.
4.19 Thoughts are not capable of self-illumination; they are mere objects of perception.
This sutra should empower one to never give undue heed to the thought-realm. Thoughts are mere objects of perception and not the perceiver. They cannot even perceive themselves. Thoughts can only reflect the Light, like the moon reflecting the Light of the Sun. In and of themselves, they have no illumination to offer—just condensed sensations of refracted Light.
4.20 The thought-realm is self-empty of perceiving subjects and objects simultaneously.
Jaganath Carrea extrapolates:
The mind-stuff can act as subject when it perceives objects or it can be an object of perception itself, but it cannot do both at once, which proves that it is not self-luminous.
I can direct the light of a flashlight on an object or on myself but not on both at the same time. At any given moment, either the object or my body will be in darkness. This is in contrast to the Purusha, which is light itself. It never knows the darkness of ignorance.
In daily life it may seem as if the mind (chitta) can be self-aware and aware of an object simultaneously. For example, I might be aware of my displeasure at the cold wind nipping my cheeks and the colorful Christmas displays I spot as I walk downtown, but not at the same moment. The subjective feeling of dual awareness is due the incredible speed at which the mind can travel back and forth between two thoughts.
[Carrera, Jaganath (2012-06-22). Inside The Yoga Sutras: A Comprehensive Sourcebook for the Study and Practice of Patanjali’s Yoga Sutras (p. 219-220). BookMasters. Kindle Edition.]
4.21 If a second mind construction were to try to perceive the first one, an infinite number of constructions could be postulated; this would result in utter chaos.
Swami Prabhavananda breaks this down in fine fashion:
If a philosopher—in order to avoid admitting the existence of the Atman—were to suggest that the mind is really two minds, a knower and an object of knowledge, then he would find himself in difficulty. For if mind A is known by mind B, then one must postulate a mind C as the knower of B, a mind D as the knower of C, and so forth. There would be an infinite regress, as in a room walled with mirrors. Furthermore, since each of these minds would have an individual memory, the function of remembering would be reduced to utter confusion. [Yoga Aphorisms of Patañjali, pg 147]
4.22 The Alaya-Consciousness of the Self does not move. Being thus free from the passing phenomenal shadow-realm, Self-Awareness occurs instantaneously. Thus does the Self dwell in Nirabhasagocara (the imageless-realm of no-shadows).
This is true Spiritual Intelligence (Buddhi). The Self-Mind is free to abstain from or to utilize thought-constructions that are under Its own purview. More than this, It never has to engage the passing shadows of discontent that afflict the saha world. It eternally and internally dwells in the imageless-realm of no-shadows known as Nirabhasagocara.