A deja vous is a feeling that one has experienced before what the individual is experiencing. I experience these feelings a lot. However, I have never understood a scientific reason for such experiences.
Some people believe these feelings of deja vous are memories of past lives (reincarnation). Other people believe these feelings are memories of dreams, which makes a little more sense. In Chapter 5 readings, the authors theorize that these feelings may be conditioned responses to stimulus generalization.
Stimulus generalization occurs when similar stimuli invoke the same response. For example, suppose someone learns to like a song while listening to that song and having fun. The learned response would be a feeling of joy from the memories of having fun. Hearing that same song at a later date and time can revive those feelings of joy. That song is the stimulus, and the response is the feeling of joy. But if a similar song inspires those same memories and feelings of joy, then that similar song is a new stimulus. Since this stimulus (the new song) is similar to the conditioned stimulus (the old song), it is a stimulus generalization.
In the same or similar sense, a stimulus generalization can inspire feelings of deja vous. If someone experiences similar situations, past feelings may surface, making the individual feel like he or she has done that and been there before.