In the Shurangama Sutra, the Buddha is leading Ananda on some exploratory questioning to reveal to him a principle.
Suppose you have a man with severe cataracts (cataracts are when the crystalline lens inside of the eye goes cloudy – so having cataracts is like looking through a cloudy window. Having severe cataracts is like looking through a brick wall from inside a dark room). You ask him, “What do you see?”
He’ll answer, “I can’t see anything. It’s all black”.
Reflect upon what that might mean. Although the blind man sees only darkness, his visual awareness is itself intact.”
Ānanda replied, “It’s true that all a blind man sees before his eyes is darkness, but can that really be what we call ‘seeing’?”
A New Translation Buddhist Text Translation Society. The Śūraṅgama Sūtra With Excerpts from the Commentary by the Venerable Master Hsüan Hua (Kindle Locations 1367-1368).
So Ananda is using his logic here – he thinks that we can only see when there is light. So when it is dark – he thinks that we can’t see – he thinks that our seeing turns off in the darkness. But is that really so? Because if our seeing nature turns off whenever there is darkness, then it would not be able to recognize “that is black”.
Using another example, when our eyes are open, we say we can see. But when our eyes are closed, we say we can’t see. But is that really so? The answer is that when our eyes are closed, we can still see – we are just seeing the inside of our eyelids (like when you draw the curtains down) blocking the light (hence why you see black) BUT our visual awareness (the ability to see) does not lapse for an instant whether in light or dark – it’s NOT dependent upon externals.
Light and dark appearing before our visual awareness are just dualistic phenomena that appear before it – 2 different ends of the same spectrum of visual phenomena.
To say we don’t see in darkness would be like saying you no longer see when you draw the curtains over the windows.
Further seeing is actually done with the mind – it is actually the mind that sees not the eyes. The eyes reveal forms but it is the mind that does the seeing. If it’s the eyes that sees, then the eyes of a corpse would be able to see.
The eyes are merely a tool – in the same way that a camera is a tool that helps us capture pictures and video of the world. But it is you who does the seeing. The eyes likewise are a tool to help us capture the images of the world, but it is our minds that do the seeing.