Because your eye actually gets its input of light upside down due to the way the lens refracts the light onto your cornea. Your brain flips it right side up in “post” if you will. The spot you see is mirrored because you are getting sensory input from pressing on your eye and cornea and your brain flips that around just like the rest of your sight.
Your eye is really a reflector (a mirror) and a big area to “catch” the reflected light. Since the eye always sees light reflected, your brain always negates this reflection by “reflecting” (inverting) it again. What you see is really a mentally reflected form of the physically reflected light entering your eye.
When you apply pressure on the right side of the eye, you are stimulating the right part of your eye. Your mind thinks this stimulation must have been reflected light that originally came from the left so you “see” it on the left.
Essentially, the primary or “true” stimulus for the receptor cells in the back of your eyes is light, but they can still perceive other things as well, such as the pressure from your finger pushing on the front/side of the eye. This causes them to report a biochemical signal that is similar to how they report light to the brain, and it’s interpreted as such. This kind of “wrong” signalling can happen via other senses as well.
Edit: also, the reason the dark spot occurs opposite from where you press with your finger is because those receptor cells are in the *back* of the eye. If you push on the right side of your eye, you “see” the spot on the left side, because you are stimulating the cells on the back-right side of the eye that would normally catch light entering the eye *from the left*. The brain then translates it exactly like that.
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