We can use thought experiments to try and get a handle on why you might think that the time traveller's predicament in knowing what she will do before she does it, is not a predicament that is peculiar to someone who travels backwards in time. For suppose that oracles exist: oracles are people who can predict the future with 100% accuracy. Now suppose that today, an oracle appears before you and tells you that tomorrow, you will decide to drive to the local pool and take a swim in the morning. It looks as though you are in a similar position to that of the time traveller: you know what you are going to do before you do it. And just as the time traveller could, in principle, know everything about what she will do when she travels back in time, before she does it, in principle the oracle could give you a complete account of what you will do tomorrow (or indeed for the rest of your life). Then just as we might worry that the time traveller is no longer in the position to deliberate about what to do, the same seems to be true of you once you meet the oracle.

What this tells us is that what raises difficulties for agency and deliberation is not travelling in time per se, but rather, coming to know what you do before you do it (and before you've decided to do it). And being a time traveller is just one way that could happen. One might be tempted to conclude therefore, that in a state of perfect knowledge (having knowledge of everything that you will do), there are no agents: there are beings that do certain things, but not beings that reason, deliberate and decide.

Still, it is all very well to conclude that in a state of perfect knowledge there are no agents, but that doesn't tell us anything about what it would be like to be in such a position. What would it be like to lack agency with respect to some choice? That seems like a valuable question to ponder, irrespective of what you think about whether there could ever be people who travel in time.

There are two cases to consider. One is a case in which there is an agent - someone who has a history of deliberating and making decisions - and that agent comes to know what they will do with respect to some future action. What would it be like, you might wonder, to be an agent that finds herself in that position? The second case to consider is one in which a being of some kind (I use that term because it is not clear whether such an entity would count as being a person or not) knows every action that he or she will perform. Thus she has perfect knowledge. If such a being is possible, then it is a being that is never an agent. Some philosophers think that if there were an all knowing being (as some people suppose a deity to be) then that being would be just like this. If a being knows everything, then she knows everything about what will happen in the future, and as such knows what she will do in the future. So an all-knowing being, whatever else it is, is not an agent. Let us call the first of these cases an impaired agency case, and the second a failure of agency case. In this blog I just want to focus on the first of these. The second of these will be the subject of a future entry.

So what would it be like to find one's agency impaired? Well we're all familiar with perfectly ordinary cases in which our agency is impaired. There are lots of things that we cannot choose to do. I cannot choose to fly, since I cannot fly. In general where I am physically prevented from doing some action, or physically forced to do some action, my agency is impaired and I will feel unfree with respect to the action. Thus I do not feel free to choose to fly. Likewise, if someone picks me up and carries me, I am not choosing to move from one location to another. I am being moved, I am not choosing to move.

Cases in which I am physically restrained in some way, however, seem appreciably different to a case in which I know what I will do before I do it. If I am tied up and cannot move, then it makes no sense for me to deliberate about whether or not to roll over. I know I will not roll over, since I cannot roll over. But here the reason I know what I will do is because I am unable to do anything other than what I know I will do. Yet that does not seem to be true in the case in which my knowing what I will do before I do it is the result of my being a time traveller or being told what I will do by an oracle.

After all, while I might know that I will drive to the local pool and go swimming tomorrow morning, clearly nothing compels me to do this, the way I am physically compelled to remain where I am given that I am tied up. It seems that it was perfectly open to me to decide to do something different with my day.  So how is it that even though I could have ended up doing something other than swimming, in fact I will go swimming, and what's more, once I know that I will go swimming I am no longer in a position to decide to go swimming, or indeed to decide to not go swimming. Once I know that I go swimming I do not seem to be free to do anything other than go swimming.

Let us consider an example. Suppose that when I was a teenager someone in their twenties came to visit me, and told me all about Zeno's paradoxes of motion. It turns out that the person who told me all of these things was an older time travelling me. The time travelling me remembers the encounter, since the time travelling me has the memories of what happened to teenage me. So time travelling me remembers being told about the Zeno paradoxes, though those memories are, of course all from the perspective of my teenage self. But the teenage self remembers quite clearly what was said in the exchange, since the experience was memorable. Now I am in my twenties and have access to a time machine. I consider travelling back in time to tell my younger self about the wonders of Zeno's paradoxes. But I remember what I did, in fact, say, and I remember that some of the explanations were not all that great and confused by teenage self. So it would be nice to offer better explanations. But since I know what I did say to my teenage self, I know that that is what I will end up saying to my teenage self.

So despite having good intentions of expressing the paradoxes more clearly, perhaps I ought to feel unfree just as if I were tied up. For I should feel destined to explain the paradoxes in the exactly the way I remember them being explained, even though I know that that is not the optimal way of explaining them. Whatever I try to do, I know that I will end up uttering the words that I remember hearing.

So what would it feel like to both want to explain the paradoxes differently, and yet to know that I will end up saying exactly what I remember my older-self saying? My colleague Dr. Nicholas Smith has given this some consideration. He thinks there are a number of ways of making sense of my psychology in the scenario just described.