We like to think only those things we experience exist. Or even can experience. What about those things we can experience that do not or never did exist? That is what illusions are. They are cases where we trick the brain into believing something happened that did not. Or tricking the brain into thinking is experiencing reality when it is missing crucially important.
Michael Bach has a nice optical illusions page to demonstrate how easily our eyes are tricked. Every sense we have can be tricked. The food industry has worked wonders in devising how to trick our senses of smell and taste. When you feel something crawling on you and look to see nothing, that is your sense of touch going haywire.
Illusions can also be dangerous. Eyewitness testimony is the worst evidence we use in the legal system. Witnesses rarely capture and retain all the details. And how they are interviewed can allow them to fill in these gaps with other evidence and skew their results to confirm that same evidence. Say the police pick up a suspect and present that photo with others to a witness. The witness picks the photo of the suspect and later out of a line up. If the suspect actually only resembles the culprit, then these two steps confirm for the witness (and the police) the suspect’s guilt even though the witness saw someone else. We attend to similarities when searching and ignore the differences. And we will go with the closest option given choices.
Inattentional blindness also falls into my illusions category. Paying too much attention to something means we have no idea about what else is happening. The train operator on his cell phone not noticing the curve for which he needed to slow down and derailed. Multitasking while operating a vehicle is dangerous because of this.
Of course, everything we experience is really the interpretation of signals to the brain. One of my favorite experiments was people wore goggles that inverted the image so everything appeared upside down. The brain just adapts to the error and interprets the picture to the desired orientation. Do not like what you see? The brain can solve that problem. Another favorite experiment was ending phantom pain in missing limbs by using mirrors to make the limb appear to exist again.
My favorite metaphor for how the brain works is Object-Oriented Programming. Different parts of the brain perform different functions. The functions adapt (are reprogrammed or reconfigured) based on the needed interpretation of the data. Of course, the adaptations are not always 100% correct. Nor do they always adapt in time not to avoid errors.