18 Free Will
Our point of entry into the problem of free will is the Grandfather Paradox.
18.1 The Grandfather Paradox
Let us first revisit the Grandfather Paradox, which (Wasserman 2017) frames in terms of the very possibility of self-defeating acts.
An act \(A\) is self-defeating if, and only if, it is an act, which if performed, then it would not be.
For putative examples of self-defeating acts, consider:
A time traveler travels back in time in order to kill his former self.
This is self-defeating because if the later self had performed the act, then he would have not been and therefore the act would have not been performed.
A time traveler travels back in time in order to destroy the technology that made it possible for him to travel back in time.
This is self-defeating because if the later self had performed the act, then he would have not been able to travel back in time to destroy that technology and therefore the act would have not been performed.
In Back to the Future III, Marty learns from the historical record that Doc Brown had been shot and killed in 1885. Marty travels back in time and saves Doc Brown’s life. Once he does that, the news that Doc Brown died in 1885 disappears from historical records and Marty no longer saves Doc Brown’s life back in 1885.
This is self-defeating because if Marty performed the act, then he would have never found out that Doc Brown had been shot, which means that he would have not performed the act.
Self-defeating acts are impossible, and the Grandfather Paradox may be thought to take this form:
- If backward time travel were possible, then it would be possible to perform a self-defeating act.
- It is impossible to perform a self-defeating act.
- Backward time travel is not possible.
(Lewis 1976) reconstructs an argument of this form in which the relevant the self-defeating act is the one Tim purports to commit, namely, to kill Grandfather. By way of reminder:
In support of premise 1:
Consider Tim. He detests his grandfather, whose success in the munitions trade built the family fortune that paid for Tim’s time machine. Tim would like nothing so much as to kill Grandfather, but alas he is too late. Grandfather died in his bed in 1957, while Tim was a young boy. But when Tim has built his time machine and traveled to 1920, suddenly he realizes that he is not too late after all. He buys a rifle; he spends long hours in target practice; he shadows Grandfather to learn the route of his daily walk to the munitions works; he rents a room along the route; and there he lurks, one winter day in 1921, rifle loaded, hate in his heart, as Grandfather walks closer, closer …
If backward time travel were possible, then it would be possible for Tim to kill Grandfather …
He has what it takes. Conditions are perfect in every way: the best rifle money could buy, Grandfather an easy target only twenty yards away, not a breeze, door securely locked against intruders. Tim is a good shot to begin with and is now at the peak of training, and so on.
Here is another reason:
Suppose that down the street another sniper, Tom, lurks waiting for another victim, Grandfather’s partner. Tom is not a time traveler, but otherwise he is just like Tim: same make of rifle, same murderous intent, same everything. We can even suppose that Tom, like Tim, believes himself to be a time traveler. Someone has gone to a lot of trouble to deceive Tom into thinking so. There’s no doubt that Tom can kill his victim; and Tim has everything going for him that Tom does. So, by any ordinary standards of ability, Tim can kill Grandfather.
But that would be a self-defeating act. If Tim killed Grandfather, then he would have never been born, much less been in a position to ever kill Grandfather.
In support of premise 2
Let \(A\) be a self-defeating act. It is a truth of logic that
- if \(A\) had been performed, then \(A\) would be.
Since \(A\) is self-defeating,
- if \(A\) had been performed, then \(A\) would be and \(A\) would not be.
So, it is impossible for \(A\) to be performed.
18.1.1 Changing the Past
Self-defeating acts fall within a broader category of acts, which, if performed, would change the past.
An act \(A\) would change the past if, and only if, either (i) \(A\) would make something happen that did not in fact happen, or (ii) \(A\) would prevent something from happening that did in fact happen.
For further putative examples of acts that would change the past, consider:
A time traveler travels back in time and manages to prevent World War II from happening.
A time traveler travels back in time and directs his former self to purchase a winning lottery number suddenly becoming a millionaire when he had not been one until now.
The problem in each case is that the act would make something that happened not happen, e.g., World War II, or it would make something that did not happen, e.g., the time traveler former self becoming rich, happen. But it is not possible for something that happened to not happen or for something that did not happen to happen. So, we have a reformulation of the Grandfather Paradox:
- If backward time travel were possible, then it would be possible to change the past.
- It is impossible to change the past.
- Backward time travel is not possible.
In support of premise 1
Choose a remote time back in the past at which no time travelers exist. If time travel is possible, then it is possible to travel to that time, which would require the past to change. The relevant time would change from containing no time travelers to containing at least one time traveler.
In support of premise 2
For each past event, we may ask whether it happened or not, but it cannot be the case that it both happened and not happened. In the example above, either the time in question in the above example contains a time traveler or it does not contain one, but it cannot be the case that it does and does not contain one.
18.1.2 Affecting the Past
One way to respond to the argument is by making a distinction between changing the past and affecting the past. We should not accept the first premise as stated. At most, we should accept the weaker claim:
- If backward time travel were possible, then it would be possible to affect the past.
An act \(A\) would affect the past if, and only if, \(A\) has a causal influence on the past.
For examples of the distinction, consider:
Suppose you travel back to the Pleistocene Epoch more than two million years ago. Say you leave an imprint on the ground, which eventually fossilizes and starts a causal chain that will trickle down to the future. You did affect the past because your imprint had a causal influence on the past, but you did not change the past. Your imprint had been there long before you were born, and you caused it as one of the outcomes of your travel back in time.
In the film Twelve Monkeys, Cole is sent back in time to find out the source of a virus, which had caused the human population to move underground. Scientists hope to identify the original strain in order to create a cure. Back when he was a kid, in 1996, young Cole witnessed a man being shot at the airport. The imagery is part of a recurrent dream of his. In 1996, Cole, the time traveler, thinks he has identified the person responsible for the virus epidemic, Dr Peters, and he follows him into the airport. He draws a weapon in pursuit of Dr Peters, and as a consequence, Cole is killed by the police on the spot. Dr Peters is able to unleash the virus, which eventually leads the survivors to move underground.
When Cole drew his weapon, he affected the past, since he set off a causal chain that eventually led to the spread of the virus. However, Cole did not change the past, since the events he caused had occurred well before he was born and had the first opportunity to travel back in time.
Here is how the distinction helps. Even if we accept the weaker premise that if backward time travel were possible, then it would be possible to affect the past, there is no reason to accept the further premise:
- It is impossible to affect the past.
For we have just seen examples of perfectly consistent time travel scenarios in which the time traveler manages to affect the past. So, we lack the means to conclude that time travel is impossible.
18.2 We are All like the Time Traveler
Even if we reject the argument against backward time travel, we must acknowledge the truth of the first premise: time travelers cannot change the past. But in that respect at least, we are not better off than Tim: we cannot change the present or the future.
Consider the proposition that I am in front of you now, which is presently true:
- I am in front of you now.
- If I am in front of you now, then it was true yesterday that I would be in front of you now.
- If it was true yesterday that I would be in front of you now, then it was necessary that I would be in front of you now.
- Necessarily, I am in front of you now.
And the concern of course is that I have no control over necessary matters. Whatever I do to make something necessary happen in the future will be superfluous, and whatever I do to prevent something necessary from happening in the future is doomed to be ineffectual.
Consider the proposition that I will write a book over the summer.
- Either I will write a book over the summer or I will not.
- If I will write a book over the summer, then it is now true that I will.
- If it is now true that I will write a book over the summer, then whatever measures I take to making sure I do will be superfluous.
- If I will not write a book over the summer, then it is now true that I will not.
- It is now true that I will write a book over the summer, then whatever measures I take to prevent it will be futile.
- If whatever measures I take to make it happen that I write a book over the summer are either superfluous or futile, then I have no control over whether I will write a book over the summer.
- So, I have no control over whether I will write a book over the summer.
The fatalist concern at this point is that we lack control over the future, Whatever will happen will happen, and there is nothing at all we can do to change that. So, if there is a difference between Tim and us is just that he had access to the historical record, whereas we remain ignorant of what the future will bring.