The human eye uses three methods to perceive and determine distance:
“The more I see, the less I know for sure.”
– John Lennon
The size a known object has on your retina – if you have knowledge of the size of an object from previous experience, then your brain can gauge the distance based on the size of the object on the retina.
“There are things known and there are things unknown, and in between are the doors of perception.”
– Aldous Huxley
Moving parallax – when you move your head from side to side, objects that are close to you move rapidly across your retina. However, objects that are far away move very little. In this way, your brain can tell roughly how far something is from you.
“All things are subject to interpretation. Whichever interpretation prevails at a given time is a function of power and not truth.”
– Friedrich Nietzsche
Stereo vision – each eye receives a different image of an object on its retina because each eye is about 2 inches apart. This is especially true when an object is close to your eyes. This is less useful when objects are far away because the images on the retina become more identical the farther they are from your eyes.
“What you see and what you hear depends a great deal on where you are standing. It also depends on what sort of person you are.”
– C.S. Lewis, ‘The Magician’s Nephew’