Chesterton's Orthodoxy, and I have to say the man was brilliant, witty, passionate, and more than a little strange. This makes for very interesting reading. Chesterton's whirlwind tour through his philosophy of life, morality, mysticism, religion, and the universe, among other things, has given me much to think about. Over the years I've come to feel that most people, including me, believe the things they believe mostly for complex emotional, social, and experiential reasons. Intellectual issues are certainly real and play some role, but I guess I've grown tired of the endless, hateful rhetoric between religious believers and non-believers. Still, Chesterton's take on things was lively, engaging, and quirky enough to get me excited again about some of these intellectual issues. Chesterton spends some time on determinism, the idea that the universe is a closed physical system (a viewpoint known as materialism) in which every event is predetermined, or fixed, based on prior events. Determinism is really the only way to think about the universe if scientific materialism, or naturalism, is true (quantum physics does complicate this somewhat, but that is a story for another day; besides, Chesterton and others argued against determinism before anything was known about quantum physics).
Chesterton reminded me of one reason that I am a Christian who believes in free will: I find determinism untenable. Certainly much of the universe is deterministic, or at least appears so. But a complete determinism I cannot accept. Humans are agents, by which I mean they have, within limits, the ability to choose or act in a way that cannot be reduced to the physical, to interactions of matter and energy. For example, if you lob a beach ball at me, there are probably a near infinite number of ways that I could respond. I could catch it. I could punch it. I could stand there and do nothing. I could catch it and eat it. I could karate chop it with my right hand. Or with my left hand. You get the idea. I believe that you could have the most advanced scientific instruments hooked up to my brain, monitoring every last bit of matter and energy, and you could still not predict which ninja move I may perform (or not).
This is fundamentally different than, say, flipping a coin, or predicting the weather. We might be tempted to think that these also are difficult or impossible to predict. But this is only due to incomplete access to information and instruments advanced enough to process the information. The way a flipped coin will land is in fact completely determined, though we may not be aware of the outcome. If instruments were available to precisely track the force applied to the coin, the angle the force was applied, the minute air currents the coin will pass through, etc., predicting how the coin will land is quite straightforward. The coin does not have a mind or agency (free will). The same is true of the weather. It would be an enormously complex practical matter to track all of the air molecules, water molecules, and energy involved to be able to accurately and exactly predict the weather, but in principle this task too is fairly straightforward. The weather is determined by prior events. When it comes to human agents though, I believe that even if we are supplied with complete information about the brain, we will still be unable to predict specific acts, such as a response to a thrown beach ball.
Of course, I don't know any of this for sure. Possibly in the future scientists will be able to perfectly understand and predict your every move before you even decide anything. But I doubt it. There is a healthy debate concerning the nature of consciousness, with strong arguments coming in from many different viewpoints, but I think the strongest case can be made for the idea that consciousness cannot be reduced to physical stuff (e.g. see David Chalmer's The Conscious Mind for rigorous arguments for consciousness having an extra-physical dimension). The idea that the mind has the ability to transcend the physical to make free decisions and feel non-physical things such as pain has strong philosophical support and squares well with each of our experiences as humans.
The thought may have crossed your mind that even if the human mind is partially non-physical, and free will is real, that this does not then point directly to Christianity. This is true. Some Christians believe in a sort of theological determinism, and many non-Christians have argued in support of free will. Really, my point here is that the Christian faith, the way that I see the world, is a reasonable one. I do not believe that Christianity (or any other worldview) can be flat-out proven to be true. But there are perfectly reasonable (and even, gasp, compelling) ways of understanding the world that fit very comfortably within the Christian faith. Many people seem to have the impression that to embrace a religious faith means to abandon any hope of having a coherent, reasonable understanding of our world and universe. I simply do not believe this to be the case, and I am thankful that Chesterton has deftly and happily reminded me of this in his strange little book.