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| Image attribution: Edward H. Adelson, CC BY-SA 4.0 via Wikimedia Commons. |
Our brains are constantly making assumptions as they perceive the world around us, interpreting and interpolating or "joining the dots" when parts of a scene, sound or other input to our senses are missing. Sometimes the brain gets things wrong. Here's an example, the checker shadow illusion. Squares marked A and B are the same shade of grey, but influenced by the variation of shades around them.
If you don't believe me, cut out the squares in your favourite photo editor and paste the pieces onto a white background. Alternatively, use the eyedropper tool to sample the RGB components.
This, along with visual hallucinations and pareidolia (seeing faces in places) are probably responsible for many ghost sightings.
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| Image attribution: Edward H. Adelson, CC BY-SA 4.0 via Wikimedia Commons. (Image edited to isolate squares A and B) |