Though he has no form my eyes saw him, his glory is fire in my mind that knows
Shankara's "First Vision" Account
It pervades the entire universe but nothing pervades it. It is effulgent by nature, and its splendor illumines everything...
The Inner Light of Crystalline Clarity
The Kena Upanishad says that the Self 'shines through the mind and senses,' which is a poetic way of saying that it is the power of the Self which allows the mind and senses to function. So the eternally conscious Self is what makes us conscious. Essentially, it is light.