Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter 4, Verse 19
Pada 4, Sutra 19
Na tat svaabhaasam drshyatvaat
The mind is not self-luminous for it is perceptible.
Anything that can be perceived either externally or internally can be classified as an object of awareness. It implies that the light of awareness can be projected onto it to illuminate it and is therefore not self-luminous. As the mind and its fluctuations can become an object of awareness, it means that the Seer that eternally witnesses the contents and modifications of the mind is the self-luminous pure subject and not the mind which is being perceived by it.
The Seer, the self-luminous pure subject, the Eternal Witness, the Atman is the Sun which illuminates the surface of the Moon, the mind.