Free will and determinism are not facts that can oppose one another. They are two types of perspectives regarding certain causes of actions. Much like the holism/reductionism dichotomy1, the question asks which of these two different but valid views of causality is correct.
In one corner, we have the determinists explaining their reductionistic view of people as conglomerations of particles following rigid laws, interacting in complex ways, and causing outcomes that necessarily resulted from their original states. (Note that not all determinists are reductionists. Some believe that an omniscient deity's foreknowledge of a person's choice makes that choice deterministic. My point about perspectives still stands.)
In the other corner, we have the advocates of free will2 explaining their holistic view of people as conscious entities who consider, choose, and carry out their actions, having the ability to take whichever course they prefer.
In both corners, we have people mistakenly believing that, because they have figured out the cause of human actions, those who believe in other causes are incorrect. They fail to take into account the simple fact that events have multiple causes.
To illustrate, imagine, if you will, five scholars debating the cause of death of a
runner-up for the Darwin Award3:
"He is dead because his life functions stopped," says the first.
"No, he is dead because his body was smashed upon the rocks," says the second.
"Both wrong. He died from falling off a cliff," the third insists.
"You are all mistaken. He died because he was absent-minded, and did not look where he was jogging," claims the fourth.
"You're crazy, all of you. He died because humans are mortal. It would have happened regardless," exclaims the fifth.
Do they not appear silly, arguing that the cause they name must be the only cause?
This same fallacy appears less silly in the determinism/free will debate, because the connections between the explanations are far less obvious. The operations of the brain, at the levels between signals and active symbols, are invisible to us. But once you see that these are two ways of looking at the same thing, the conflict between them vanishes.4
The course a person winds up choosing is the only one they can choose, but it is made the one path by virtue of the fact that they have chosen it. The mechanical interaction of matter does not force a person to choose against their will; it enables a person to have a will and to choose in accordance with that will.
1. For a masterful elaboration on this dichotomy, and its
application to music and consciousness, read "Prelude, Ant Fugue,"
a pair of dialogues by Douglas Hofstadter that appear in Gödel,
Escher, Bach: An Eternal Golden Braid and in The Mind's I.
2. Another version of "free will" (which I will not explore in detail here) is proposed by some who feel that, because a mind based wholly in matter and operating through set physical principles cannot have free will (a point with which I disagree), a dualist conception of a soul that is not bound by physical reality is the only venue for "real" free will. I find it less than likely (or more accurately, "less than possible") that a soul that is not bound by physical reality can have any influence over the workings of a brain that is bound by physical reality.
3. The annual award to the person who does the gene pool the biggest favor by removing himself5 from it in the stupidest way possible. The Darwin Awards site.
4. "The conflict vanishes" sounds like mystical hand-waving, does it not? Yet the conclusion sheds light upon "real life" matters. Have you ever heard someone try to excuse the behavior of a criminal, claiming that it was caused by an abusive childhood, and therefore the criminal should not be held liable for his or her acts? Or the opposition, trying to declaim causes other than conscious choice? Seen from a perspective that accommodates multiple causes6, it becomes clear that while childhood experience could easily be the cause, the decision to commit the crime was still a conscious choice, still a product of the criminal's free will (assuming it was voluntary). Culpability is preserved.7
5. The masculine pronoun is appropriate here; every winner has been male.
6. Aristotle assigned four types of causes to each event: The material cause (the third domino in the chain fell because of gravity); the formal cause (it was part of a chain of falling dominoes, each of which caused the next to fall); the efficient cause (I pushed over the first domino); and the final cause (it fell for my entertainment). We can name still more causes (e.g., because the second domino fell, or because I have enough uncommitted time to set up dominoes) of debatable faithfulness to these four categories. Note that I have taken liberties with my examples of Aristotle's causes, in order to maintain illustrative power while applying them to events rather than (as the classical examples apply) to objects.
7. Quotations: 3