“Then you would have acted to change history, but you would have replaced one history with another. You would not be able to know what past version of history you would have altered, because that history would never have happened. Nor would you be able to know what you had done in the past, or that you had done anything at all in the past, or that you had even been to the past. You would not be able to compare histories in your mind; if you entered the causality violation device knowing this, then you would realize that you would be trading one set of memories for another, and that there would never be any evidence, in your mind or in the world, that you had done this.”