Why do people see illusions?

Answer 1

Illusions are common phenomenon: It's when your eyes and brain don't communicate properly and you see something that is not there. This can be the result of hallucinations from drugs, exhaustion, poor health, etc.

Answer 2

The human brain is designed in such a way as to fill in blanks when not all data is immediately available.

Ordinarily, this provides a more useful picture of what we think we see, by interpolating data to cover things we don't actually see.

As a "for instance," the part of your retina where the optic nerve is attached right behind it, is actually not able to see or record data. If we were seeing what we really were seeing, we'd see an annoying little blank spot in the middle of our field of vision. Instead, our brains fill in the blanks, and we see what appears to be a cohesive image with no little missing spot.

But this trait, which makes our vision (and other perceptions) more user-friendly and usable, can be exploited.

What we think we see, or remember seeing in any incident, is usually a recorded "best guess" that's made after the fact. It looks seamless and accurate to us, but is actually a "guess" assembled out of incomplete information and perception.

That's almost always good enough, but good enough isnt' always 100% accurate.

So, basically, we see what part of our brains expects to see, based on limited data and best guesses, so sometimes, we fall for illusions.