If you have ever walked into a completely unfamiliar and strange situation and felt that somehow, somewhere in your life you had done it all before, then you have experienced deja vu. Deja vu, the strange feeling that one has been someplace or seen something before, is a bizarre mental phenomenon that proves that the human memory can sometimes play tricks on itself.

Because of its mysterious nature, psychologists and others have been fascinated with deja vu. Some claim that it is evidence of reincarnation, arguing that during deja vu the subconscious recognizes an event that occurred once before in a person’s former lifetime. Others defend their skepticism by arguing that deja vu happens when a person experiences something for the second time after merely forgetting their first experience. But the plain truth is that deja vu is not only real, but is as normal and commonplace as it is mysterious.

“Everyone reports having experienced deja vu at some point in their life,” said professor Patricia S. Churchland, chair of the UCSD philosophy department. “In fact, I would be surprised if I met someone who told me that they had never experienced deja vu.”

And it’s true. We can all probably remember a time when we thought that something looked familiar, knowing for sure that we had never seen it before.

“Memories are normally accompanied by a certain feeling of familiarity,” Churchland said. “But the brain is an imperfect machine, and it has a lot of jobs to do. So occasionally an inappropriate feeling of familiarity accompanies a situation in the absence of a memory. Deja vu is nothing more than an imprecision in memory, like a mental hiccup.”

Those who experience deja vu, however, like to think that their feelings of familiarity come from real memories, not just mental imprecision. But these memories have to come from somewhere besides personal imagination.

Some suggest that these distant memories come from past dreams. According to this theory, deja vu occurs when the subconscious recognizes a real event to be similar to an event in a past dream. After sleep, the conscious mind usually forgets the events that take place during a dream. But perhaps the memory of dreams is stored somewhere deep within the subconscious mind, and during deja vu, the memories resurface.

Another theory, formulated by the father of psychology, Sigmund Freud, suggests that a guilty subconscious can sometimes force the memory to forget certain events from the past, or to “remember” events that never happened at all. According to Freud, deja vu occurs either because a person wants to remember something that never happened, or because a person wants to suppress a memory of an unfavorable event.

Clairvoyance, the ability to see into the future, and precognition, the cognizant knowledge of an event before it actually occurs, have also been cited by psychologists as being responsible for the deja vu phenomenon. The psychologists who support this theory have very little evidence to maintain their argument. They draw from highly anecdotal sources claiming that deja vu must only be part of the cognitive process because it is linked exclusively to vision and not any other sense. And vision is connected directly to the cognitive areas of the brain, unlike the sense of smell which is linked directly to memory.

Unfortunately, because of the complexity of the human brain, it is impossible to prove or disprove any theory about deja vu. Perhaps different people experience deja vu for different reasons, since individuals can make up their own minds. What lies beneath this mysterious phenomenon may then be more a matter of personality than neurology.