We introduced the story in our first installment, bringing it to the point at which they begin working on a time machine. There is, however, a significant problem in the story already: a major part of their present situation is the result of the fact that David saw himself in that video. The obvious impact is that they have the motivation that comes from the belief that they have already succeeded. That is reflected in the response to the question of whether they were going to build a time machine, “I think we already did build it,” and again when Jessie “Jess” Pierce is added to the group and she challenges them that they already know that they can make the trip because they already have the proof that they will have done it (overlooking that they might all but David have died trying to make it work, as only he is seen in the video).
This is a variant of a predestination paradox, in which something in the past is caused by a future event which is in turn caused by the past event. In this case, because David has video of his appearance in the past he makes the effort to get there and also makes leaps he might not otherwise have made had he not had this proof. Yet it is more complicated than that. Because he has Jessie’s keys in his hand, she has a basis for pushing her involvement in the group when they are identified; that happened simply because he had access to her car and used it just before his abrupt departure on that final trip, but he had the car because of their relationship, which happened because she became involved in the time travel project. Yet there is an even bigger predestination paradox here: the light switch.
David found the lock box in the floor because he heard the solenoids when someone flipped the switch; he had them flipping the switch because in the video he saw his own hand reach for that switch. Why, though, did he do that in the video?
The switch was either on or off. If it was off, then logically he must have been turning it on in order to have light in the basement; if it was on, he was turning it off in order to unlock the lockbox. However, we learn eventually that David’s father was already in the basement at that moment. If someone flipped the switch, the lights would change status–either from off to on or from on to off. Yet when the switch is off, the lights are off, and while it is likely that there was another light in the basement on a separate circuit, when David’s father came downstairs he would have turned on the light at the top even if he then turned on another light below (and when David was looking for the lockbox, why did he not ignite the other lamp?). So David probably found the lightswitch in the on position, and probably would not have doused the lights before descending the stairs (there is no sign of a flashlight, and the arrangement of the cluttered basement ten years ago is likely to be a bit different than he remembers)–and the light is on when he finds his father down there. That means that there is no reason for David to reach for the lightswitch unless he does it so that he will see himself do it.
This makes perfect sense in a fixed time story; this, though, makes itself very clear that the time travelers are changing history, and changing their own history. For such a paradox to work in a replacement theory story we have to find an original cause. Perhaps the original David, the one who did not know his actions were going to be seen by his younger self on the video, reached for the switch to ignite the light and then saw that it was lit. The fact that that action was important was incidental.
Besides, there is a much bigger problem with that segment, which we will eventually address, that makes any effort to resolve that paradox moot. We have a lot of other ground to cover first, though.