The degree of circumstance, the ratio of memory to forgetfulness, determines whether a dream will be a recognized, fulfilled prevision, or the vaguely, effective source of the weird deja vu feeling. No doubt some experiences vanish so completely as to leave no trace on the sleeper's mind. Probably less than one percent of our previsions escape final obliteration before we wake. When we arrive at the events concerned in the vanished majority, they, of course, cannot impress us as anything familiar. Nevertheless, there are notably frequent instances of deja vu, in which our recognition of an entirely novel event is a feeling of having lived through it before, a feeling which, though vague, withstands the verbal barrage from the most impressive corps of psychologists. If deja vu is an illusion, then peculiarly, it is a most prevalent mental disturbance affecting even the most level-headed people.

Chauncey Depew, one-time runner-up for the Republican Presidential nomination, was attending a convention at Saratoga, where he was scheduled to nominate Colonel Theodore Roosevelt for Governor of New York when he noticed that the temporary chairman was a man he had never met. After the preliminary business affair was finished Depew arose and delivered the convincing speech that clinched the nomination for Roosevelt. If Depew had told any academic psychologist that he had a weird feeling of having lived through that identical convention session at some time in the past, he would have been informed that he was a victim of deja vu. But the famous orator felt more than vague recognition for the scene. He remembered exactly when he had lived through it before, and he had something to prove he had.