Na tat svabhasam drsyatvat.
Na = not; tat = it; svahasam = self-illuminative; drsyatvat = because of its being an object of perception.
Nor is it self-illuminative, for it is perceptible. (IV-19)
This and some of the subsequent sutras confirm the theme of previous sutra indicating the fact that our mind shines with the light of consciousness. It does not have its own light just like moon which shines with the light of the sun.
Perception of various objects in the day happens due to the light of the sun and in the night it happens due to the light of moon and stars. How do we perceive the objects of dream world? The pure light of consciousness is the answer. How do we make the occasional statements like "my mind is not functioning properly today", "my mind is dull today", "my memory is failing" and so on? Who 'sees' these facets of mind? It is Purusa, the pure consciousness while the mind in itself is jada, inert.