At the same time as we try to get clearer on what free will is, we'll also be trying to get clearer on whether we have it.
Worries about whether we have any free will arise from several different sources.
One sort of worry starts from the premise that the laws of nature and the past causally determine that you will act in the way you do. This seems to show that you're not free to act in other ways. This worry concerns the relation between free will and determinism.
A second sort of worry starts from the premise that there is only one "real" future. Perhaps someone already knows how this future will turn out. (For example, perhaps God knows this. Or perhaps some oracle with a crystal ball knows this, or a time traveller who's come here from the future. Or perhaps we already have a problem if future historians looking back on the present day will know how things turn out.) Or perhaps they don't.
Even if no one now knows or is able to predict how you will act tomorrow, maybe there's already some truth about how you will in fact act tomorrow. And if you will in fact act in certain ways tomorrow, it seems to be already true today that you will act in those ways tomorrow. This can seem like it would also deprive of you of the freedom to act otherwise: it's impossible for you to act otherwise tomorrow, because it's already true that you will act in the way you will. This is what's called a fatalist worry.
One way to respond to these worries is to deny their respective premises. That is, we might deny that the laws of nature and the past causally determine your actions; and we might deny that claims about what you will do tomorrow are already true today.
Another way to respond to these worries is to accept the premises (for the sake of argument), but to argue that no dire conclusions about free will follow from these premises. We will primarily be exploring responses of the second sort.
The best way to define Determinism is as follows.
Certain laws of nature are such that, given a certain past, those laws are compatible with only one subsequent future. Any world with that past but a different future would have to have different laws. We call laws of this sort deterministic laws. Given a certain past, these sorts of laws require the future to go in a unique way. Any two possible worlds, if they started off the same way, and both had the same deterministic laws, must continue in the same way.
Other laws of nature are compatible with more than one subsequent future. We call these indeterministic laws.
We will understand Causal Determinism, or Determinism for short, to be the thesis that all of the laws of nature that govern our world are deterministic laws.
It is not clear whether or not the laws that govern our world are deterministic laws. Our best theories of subatomic physics certainly do not appear to be deterministic. They involve probabilities in ways we can't eliminate. However, it is philosophically controversial how the probabilities in those theories should be interpreted. Hence, one cannot easily say whether or not our best theories of subatomic physics show our laws of nature to be indeterministic.
In this class, we won't worry too much about the kind of indeterminacies that our current theories of subatomic physics introduce. Even if those indeterminacies do show that our laws of nature aren't deterministic, those indeterminacies would all be at the microscopic level. At the macroscopic level, our laws of nature would still be so close to being deterministic that it would make no practical difference. People often start worrying about Causal Determinism like this: Suppose I pull out a gun and shoot you with it. Now if I could not have avoided doing that, if the laws of nature and the way the world was before I was born causally determined that I would do that, it looks like I'm not responsible for my action: how can I be responsible for something that I could not have avoided doing? Have we really addressed that worry about moral responsibility, if it turns out that the laws of nature did not 100% determine that I would shoot you, after all? They only made it 99.9999999999% likely that I would shoot you. There was a minute chance that I would have done something else. Is that enough to make me morally responsible for shooting you? According to our theories of subatomic physics, there is also a minute chance that when I stick a pin in a balloon, all the air in the room will rush into the balloon and make it get bigger. There is even a minute chance that when I drop a match in a bucket of gasoline, the gasoline will freeze solid instead of burning. The minute chance that I would not shoot you just then is cut from the same cloth. Is a minute possibility of that sort enough to show that I wasn't forced to do what I did, and hence, that I'm morally responsible for it?
Even if you think Causal Determinism is false about our world, still, you will learn a lot about our concept of free will by investigating what would follow if Causal Determinism were true. So that's another reason not to get too worried about the kinds of indeterminacies that our current theories of subatomic physics introduce.
Some philosophers hold the view that Determinism and free will are incompatible with each other. They think that if you've got one, you can't have the other. We call these philosophers incompatibilists. Other philosophers think that Determinism and free will are compatible. They think that it's possible to have both. We call these philosophers compatibilists. We can further sub-divide the incompatibilists and the compatibilists as follows: