The term deja vu was coined in 1876 by the French philosopher Emile Boirac. It is the overwhelming sense that you have already experienced something before. It translates literally from French as 'already seen'. According to research, about two thirds of us experience at least one deja vu in our lifetime, yet very little is known about what causes it. A group of scientists have studied the strange case of the man with 'chronic deja vu'.