It arrives without warning and in the most ordinary places. You are standing in a kitchen that is not yours. Someone across the table lifts a cup, and before the cup reaches the mouth you know the sentence that will follow, and the sentence follows. The light on the wall is right. The chair you have never sat in has the give you expected. For four or five seconds you are not experiencing the afternoon so much as confirming it, and then the correspondence breaks and the room goes back to being new. People call this déjà vu and treat it as a hiccup of the brain. It is recognition. You have been through this moment before, in a passage through history you can no longer give an account of, and for those few seconds experience ran ahead of events because it had somewhere to run from. To see why this is possible you must separate two things that daily life keeps fused. The first is historical sequence: the world as a succession of complete states, each holding its people, its weather, its evidence and whatever business the previous state left unfinished. The second is personal continuity: the memory and the conduct of the person who passed through those states. History is kept in the sequence. Memory is kept in you. Under ordinary conditions the two agree so exactly that you never notice they are separate, and you assume the past has gone the way a burnt letter goes. It has not. Yesterday is still in the ancestry of today, fixed and complete. What has changed is only that it is no longer current, and that your means of reaching it are limited. When the sequence is altered and your memory is not, the two fall out of agreement, and the consequences depend entirely on the scale of the disagreement. Consider a traveller who enters an earlier state. He does not step into a recollection. He steps into a Tuesday. The letter has not come. The train has not left the platform. The man whose death notice he read last week is at a counter counting out coins for bread, and nothing in his manner concedes that from the traveller's position he has been dead for a month. Watching him costs the sequence nothing. The traveller may stand in the doorway until the baker closes and the world will take no notice. But he might speak. He might mention the train. He might keep the man talking for the four minutes that decide the matter, and then the man lives, and every letter he writes afterward and every child he fathers and every debt he incurs must be accommodated somewhere. History does not waive this because the traveller was fond of him. It opens a branch: a sequence sharing every state up to the moment of interference and then running on its own, in which the man grows old, while the parent sequence keeps its funeral, its hymns and the particular rain of that morning, undisturbed. Branches can be rejoined, but only by reconciliation, and reconciliation is a decision, not a procedure. Two contradictory outcomes cannot govern the same subsequent events. Somebody chooses which account continues and what becomes of everything built on the other, and there is no tidy method for returning a husband to a woman who has already learned to live without him. The most dangerous operation in the whole structure is the revision of the prime sequence itself, the accepted ancestry of the present, after a great many people have lived through it. The archives comply at once. The books reprint themselves. The photographs find their new shadows. Public records settle into the amended account as if they had never held another. The witnesses do not comply, or only partly. They retain impressions acquired elsewhere in the structure, and those impressions are what we have come to call Mandela effects: a spelling that no edition now supports, a famous sentence with one word wrong, a death remembered at a date the registers deny. These are not failures of memory. They are the only evidence of a revision that survives the revision, and they tell you that someone stood where you are standing and chose a different past for everyone. Déjà vu is the same separation at a smaller scale, and it is produced by a different kind of passage. A Mandela effect concerns history: something the accepted record no longer contains. Déjà vu concerns the next few seconds: the arrangement of a room, the turn of a conversation, the movement a hand is about to make. The passage that produces it is death. Death ends the body's participation in its branch. The burial stays there, with the people who attended it, and they are not deceived; they will grow old along that sequence, and the grave will always hold what they put in it. The observer does not stay. His continuity resumes at an older branch, at the earliest viable position the structure can offer, and between departure and return lie two things: passage and judgment. The passage is the river. The Styx runs along the border of every terminal state and is not crossed unaided. Charon waits at the near bank and takes his fare, which is why the living have always closed the eyes of the dead over copper or set the obol under the tongue. The coin is paid from the world being left, by the hands of the people who loved the dead man, and it is the last thing they do for him that matters on the far side. Those for whom no one pays, the unclaimed and the unnamed, wait on the shore in a crowd that does not thin. Charon asks nothing beyond the coin. He does not want to know what you did. Anubis wants to know nothing else. He stands at the interval before return, and his jurisdiction is the observer's accumulated conduct across every branch, including conduct that no surviving history records. Rewriting a sequence erases the act from the world's ancestry. It does not erase it from the one who performed it. A man is not acquitted because the history containing his crime was displaced, and a kindness is not void because nobody in the receiving world will remember it. The verdict governs the conditions of return: to which branch, to what circumstances, with what, if anything, retained. Almost nothing is retained. The returning observer arrives without an orderly account of where he has been. Names go. Dates go. Whole decades go. What survives, when anything does, is recognition without explanation: a room known before the door opens, an aversion with no recoverable cause, the oppressive familiarity of a conversation that is apparently happening for the first time. Déjà vu is the moment the returning observer reaches a surviving point of correspondence, a place where the older branch and the lost one still coincide closely enough for the fragment to catch. It lasts only as long as the coincidence does. Then the branches diverge again, the sense of foreknowledge collapses, and the afternoon resumes. Most of the previous passage will never be accessible. What remains is brief and incomplete, a handful of seconds in which a cup is lifted and a sentence is finished for you, which is enough. It is enough to tell you that these seconds have already been lived, that you have crossed at least once, and that someone, in a world you can no longer reach, put a coin under your tongue so that you could.