At some level, everything is changing. The passage of time if nothing else, ensures that no single thing that can be perceived is ever the same as it once was, simply because it is older than it used to be. Iron rusts, plants grow, people change. Everything is always different from how it ever was. To paraphrase Heraclitus, the only thing constant is change.
Human perception is either blind or acclimated to most of these changes. The fine layer of dust that has settled on a chair does not prevent us from recognizing the it. Even though it is strictly speaking, not the chair you remember, nothing you are going to use it for is likely to be affected, so the change goes unnoticed, ignored, or is processed without conscious thought. This nearsightedness is not entirely a bad thing. Indeed life would be impossible if a person had to re-evaluate their situation each time a particle landed on the chair. Ignorance of change is the core of the concept called constancy, the belief that certain things will not, or cannot change. Generally, the evidence suggesting something will not change is that, so far, it hasn’t changed. Deviation from this perception of constancy is generally considered mental illness.
Considering how much is occurring that we don’t perceive, how sure can you be of what you do perceive? Familiarity is an illusion. As a person’s body replaces old cells with new ones, they become, physically, a different person. Mentally, new experiences have changed them, yet the illusion of familiarity still brings strong emotions, and gives us a foundation with which to interact. Does perception show us reality, or merely allow us to cope with it?
In order to live in our world we need to continually test our perceptions of what is real against the perceptions of others. Reality is a matter of consensual validation. We agree that a tree is a tree; and a chair is a chair. Our exact interpretations of these objects may differ somewhat, but we agree on the generic class enough to communicate meaningfully with each other.
In this manner, we create reality. We have our own perceptions of things, but look to others for confirmation. Like an uncertain math student, we compare answers with the person next to us. If our answers match, it affirms our knowledge, and we proceed with confidence. If they differ, we rework the problem until the answers are made to match. Through use of this simple system, we determine what reality is, but we would do well to remember another lesson learned from using this system in class: Finding the same answer often means we’ve made the same mistake.