David Lewis’s essay in The Paradoxes of Time Travel has many underlying implications on the ongoing philosophical discussion regarding free will. Here, free will can be defined as a power to make a choice independent of external agencies. Of course, all of our choices are made because of some instance of cause and effect - e.g. because one is inquisitive, one chooses to learn more about the world so one chooses to go to university - but the question lies in the extent of which these external forces influence our choices. In this case, Lewis convincingly uses arguments concerning time travel, which are backed by modern day theories in physics, to take on a compatibilist position in the free will debate, arguing that although a fixed and predictable timelines exist, otherwise known as external time, the fact that we have, as he calls it, a personal time gives us the freedom of choice that we so desperately need and strive to argue for.
For a time traveler, Tim, the personal time is the time on his wristwatch, or the time it takes for his beard to grow regardless of what is happening in the external time. If there is no such thing as personal time, Tim would presumably shrink in age when traveling back in time or grow old rapidly when traveling forward through time. Lewis does not consider this, but it helps distinguish between the two types of timelines as one is traveling on the plane of the other. Lewis describes the shape of the personal timeline when traveling towards the past as a zigzag streak, towards the future as a stretched-out streak and towards either way instantaneously as a broken streak. The differentiation between the personal and external timelines may seem trivial at first, but that is exactly where our free will originates. The determinist view would need to deny any view which states that we have our own time, meaning that it would deny any non-classical form of physics.
As with any good philosophy, it is always best to create and examine the extreme form of the example and that is exactly what Lewis does by supposing that Tim has an unstoppable motivation to go back in time to kill his grandfather in the grandfather paradox. If we were to rule out all factors stopping him from killing his grandfather, there would be no reason for him not to do so. Although there would be a contradiction in Tim’s past if he was to pull the trigger, all it would do is fortify the multi-verse argument - meaning that there are many more different four-dimensional worlds - as there is absolutely no way that anything could stop Tim. Just as there are many different personal timelines, there are many different external timelines. Our choices help choose which paths we take on these external timelines. The determinist would argue that there is only one external timeline and it would be impossible to kill the grandfather as it would necessarily result in a contradiction; it would destroy any possibility of free will as every time Tim tries to kill the grandfather, he would always need to somehow survive.
Lewis’s argument, however, does rest on the notion that time travel is indeed possible, but his conclusions imply that we should be able to travel between these external timelines meaning that we can travel between the various choices we have made. His argument finds that free will exists because the external time is unrelated to our personal actions made within our personal time. His conclusions and the possibility of time travel, are, however, supported by most non-classical physical theories including the many forms of string theory.
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