One way to understand the tension surrounding our ordinary intuitions about whether or not agents have free will is as follows: below, are three intuitive theses. Individually, they each seem plausible. Yet they cannot all be held consistently together; one of them must go. The problem, then, is in deciding which seemingly intuitive thesis we should reject. Here are the three theses:

    (1) Free Will Thesis: Agents sometimes act freely.
    (2) Causal Determinism Thesis: Every event is causally determined to come about by some other event. Or: every event is caused.
    (3) Incompatibilism Thesis: (1) and (2) are incompatible.

Thesis (1) seems plausible for at least two reasons. First, we often feel free. When you are standing at the coffee shop trying to decide between an Americano or a Frappaccino, you certainly feel as if you get to choose which one you want. When you are deciding whether you should study for that Chemistry exam or go to a party, you certainly feel as if you are weighing the pros and cons of each, figuring out how much you really care about the exam, whether it is worth cramming for in any case, or whether or not that cutie you've had your eye on will be at the party, etc. At times, you deliberate carefully and consciously, and seemingly make a decision between more than one option. You sometimes feel as if you act freely, and this is one reason to think that (1) is true. In addition, however, is the fact that we treat ourselves and each other as if we are free. We praise and blame, reward and punish, all on the assumption that we sometimes act freely. For example, if you get the sugar-loaded Frappaccino rather than the (relatively) healthier Americano, you may hold yourself responsible when those love handles start getting less lovable. Or you may reward yourself after a job well done if you study for the Chem exam instead of going to the party, and end up acing the exam. You also praise or blame others for things they purportedly did freely. A lover who lies to you is blamed for being untrustworthy. A friend who skips a party to help you out when you're feeling down is praised as being a genuinely good friend. Even our legal system is based on the assumption that (1) is true: criminals are punished for breaking the law; innocents are exempted from wrongdoing; heroes are given medals, etc. You are even given grades in school that purportedly reflect how well you have done, letters to represent rewards and punishments for studying well or poorly, degrees awarded to those who preform decently, which are eventually the basis for getting a job, etc.  Indeed, in any of these cases, if it is discovered that someone had no control over their actions--e.g., if a criminal is deemed insane, or is forced at gunpoint, or if a student is shown to be mentally incompetent, or under the influence of hallucinogens that were involuntarily taken--we tend to no longer hold them responsible for their actions. All of our systems of reward and punishment seem to assume that we sometimes act freely, and this is a second reason to think that (1) is true.

Thesis (2) seems plausible because of our common sense, pseudo-scientific view of the world. For any event, we seem to think that there was something that caused that event. Take a rock falling off a cliff, for example. It would be odd if, in answer to the question what caused the rock to fall?, someone replied "Nothing. It just fell." We seem to expect the world to be ordered in such a way that there is an answer to questions such as what caused to rock to fall?, even if we may not know what that answer is. For example, perhaps the rock was precariously placed at the top of the cliff and a gust of wind caused it to topple. And we can ask of the wind: what caused it to blow in just the way that it did? The answer might have something to do with the weather patterns for that day, the high and low pressures in the surrounding area, etc. And we can ask of the precarious position of the rock: how did it get there? The answer might be that someone carefully put it there, or that it had been there awhile, ever since the cliff was formed, but that sediment had eroded beneath it because of the wind and rain, etc. For nearly any event we can think of, there seems to be a reason we can give to explain why this event, or a cause that explains how this event came about. Science seems to assume that this is true, and our ordinary intuitions about how the world works seems to assume this as well. So this is why (2) seems true.

Thesis (3) seems plausible because of the following bit of reasoning. Take any event, such as your decision to go to class instead of sleeping in. And let us suppose that this 'decision' is one of those events where you act freely, making (1) true. Yet if (2) is also true, then there should be something that caused you to decide to go to class rather than sleep in. Suppose the cause of you attending class instead of sleeping in was your desire to do well in school (to get good grades so that you could graduate so that you could get a good job, etc.). But given the truth of (2), there should be something that caused your desire to to do well in school. Suppose the cause of your desire to do well in school was instilled in you by your family, or by your desire to get a good job eventually. But then, again, if (2) is true, then something caused these desires, and something caused those things, and so on. So, eventually, we get an explanation of the cause of you 'deciding' to go to class that looks unlike anything we would intuitively call 'free will'. Put more explicitly: we intuitively think that acting freely means acting uncausedly. But if acting freely requires not being caused to act, then this will violate (2). So, it seems as if (3) is true.

Theses (1), (2) and (3), although each individually plausible, cannot all be held consistently together. So one of them has to go. Your position on the Free Will debate will depend on which of the theses you accept and which one you reject. Below, briefly, are three positions on the Free Will debate.

    Hard Determinism: Accepts (2) and (3), rejects (1).
    Libertarianism: Accepts (1) and (3), rejects (2).
    Compatibilism: Accepts (1) and (2), rejects (3).

There is much more to be said here, of course. We will discuss what we can in class.

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Related Resources:

Stanford Encyclopedia of Philosophy entry on Free Will.
Stanford Encyclopedia of Philosophy entry on Free Will and Omniscience.