From its being unable to cognise both at the same time.
In plain words: The mind cannot notice an object and notice itself at the very same moment. This shows the mind does not shine with its own light. The Purusha, the true Self, is what gives it light.
If the mind were self-luminous it would be able to cognise itself and its objects at the same time, which it cannot. When it cognises the object, it cannot reflect on itself. Therefore the Purusha is self-luminous, and the mind is not.
Namaste.
A calm guide to life, the mind and the old teachings.