The rerun

You turn into a street you have never visited, in a city you have never lived in, and the whole frame lights up: the blue door, the bicycle, the exact slant of the light. You have been here before. You also know, with complete certainty, that you have not.

Most people know the feeling. In Alan Brown's 2003 review of a century of research, about 60 percent of surveyed people reported at least one episode, and the experience gets rarer with age. It tends to show up under stress and fatigue, and Brown sorted a hundred years of explanations into four families: two cognitive processes briefly out of sync, a small neurological misfire, a memory you cannot quite retrieve, and attention that glanced at the scene before you properly looked.

The memory family has produced the cleanest lab results. In 2012, Anne Cleary's lab at Colorado State built virtual-reality scenes and later showed people new ones. Some of the new scenes shared their layout with a scene viewed earlier, same arrangement of elements in different dressing. When people failed to recall the earlier scene, the lookalike still felt familiar, and deja vu reports rose with it. The more the new scene's configuration matched a stored one, the stronger the effect, precisely in the cases where recall failed. One working explanation: memory keeps a sketch of a scene, not just a list of scenes, and when a new place matches a sketch it cannot name, it raises a flag without a label.

But what about the eerie part, the sense that you know what happens next? In 2018 Cleary and Alexander Claxton tested it directly. They gave people navigational paths resembling ones they had seen but could not recall, asked them to predict the next turn, and measured. Deja vu did not make anyone better at predicting. The resemblance raised reports of deja vu, and the state came with a stronger feeling of knowing the next turn, but the guesses stayed at chance. The premonition was real as a feeling and useless as a forecast. One working reading: the flag is so confident that it borrows authority it has not earned.

That leaves the everyday experience in a funny spot. The feeling is doing honest work. It really is a match detector firing. It is just firing on a match it cannot show you, and then dressing the misfire up as a story about the future.

You can watch the machinery half-work. Next time the street lights up, stop and try to name what comes next. Not the feeling, the specifics. Which way does the path bend? What is behind the blue door? The feeling will be sure. The answer will be a coin flip. The gap between those two is the whole phenomenon, measured from the inside.