Chaos Theory says that "Free Will" is a moot issue.

"Free Will!"  Sounds so good, doesn't it?  Everyone wants to think that they have complete control over their thoughts and actions.  Sorry, no.

Scientists have found that using an fMRI, they can actually see a decision being made long before the person making that decision recognizes that they have made it.  Ok, maybe that doesn't mean you don't have Free Will.  maybe it simply means your thoughts are being made at a level below where you realize it and only later do they arrive at a conscious level.

How about the argument which says:

You are the sum total of all of your genes plus all of your experiences.  If you could start again with the same genes and have the exact same events happen to you, you would end up in the same place and make the same decisions.

Interesting argument, but can this ever really happen?  Is this even a realistic scenario?  Certainly not.  We know that the exact same series of events can never happen exactly the same way, especially if they are complex events.

This ties into Quantum Physics and Indeterminacy.  If you haven't learned much about Quantum Physics, then watch this entertaining video about the famous "Double Slit experiment.  This talks about particles appearing as both waves and particles.  Related to this is also the topic of "Super Position", where a particle or wave can be in multiple states at once.  As if this isn't enough, let's also look at the issues brought up by "Delayed Choice".

So, with multiple possibilities existing simultaneously, can we ever really know the complete state of a system, enough to really predict the outcome, especially a system, such as the brain, made up of billions and billions of neurons?  Well, this actually should make you think of "Chaos Theory".  Here's a video showing how even a very simple system of two pendulums will produce what appears to be "random motion".  Notice that even a small change in the initial position of 1 degree will produce a wildly different result!  Here's a video of a real world example of the same experiment.

So, what have we learned?

1. The brain is performing actions, creating thoughts, and reacting to both itself and the outside world in ways about which you can not have any knowledge.
2. At the most basic level, all of nature is made of probabilities, not particles.  These probabilities can interact with each other in unpredictable ways.
3. Chaos theory shows us that even the smallest change in initial conditions can make even simple, Newtonian, systems perform in unpredictable ways.  If this is true of even simple systems, then we are incapable of imagining the complexity of any "real world" system.  (This is, incidentally, why even today, with very powerful super-computers, we still can not predict weather very reliably.)

So, when you take all of this into account, do we really even need to discuss the topic of "Free Will" any more?  No, you don't really have "Free Will".  Everything you think or do is completely a result of the starting conditions of your brain, followed by all of the events and thoughts which came before the action.  But, does it matter?  NO!  It doesn't matter at all, because there is no way that you or anyone else could ever reliably predict any outcome, even if the entire series of events were known and all of the connections inside your brain were known.  The organ which is contemplating "Free Will" is such a complex system, based upon other incredibly complex systems, and ultimately fabricated out of probability functions, such that for all practical purposes, it may as well be "Free Will".

The "Free Will" discussion is ultimately seen as a senseless discussion by those who understand more of the ways in which the world actually works.  To even have such a discussion, is to admit that you have no idea of the complexity of the world.  Now we understand more about Quantum Physics and Chaos Theory, we can clearly see that the "Free Will" discussion is entire moot.