Oh goodie. Another free will discussion.
Ultimately, the realization of whether free will exists or not does not come from arguments that we are machines or God's omnipotence. It comes from the realization that the very concept of free will is incoherent. Free will does not exist for the same reason that four-sided triangles don't - the definition is a contradiction of terms.
Let me demonstrate. There are two types of systems that we know can theoretically exist - random systems and deterministic systems. There may be more - I'll get to that later.
In a completely random system, effects do not depend on any prior cause. This makes things that happen in a random system "free" in the sense that it's impossible to tell what will happen in the future, but all basis for order and rationality are obviously missing in such a system. Obviously, neither our universe or our minds are completely random for this very reason.
In a completely deterministic system, all effects depend fully on prior causes. This means that, given perfect knowledge of the system, one can perfectly predict the behaviour of that system. Given partial knowledge of the system, one can approximate the future behaviour of that system. The approximation will improve with the available information. This fits rather well with both our universe and our selves, although scientists tell us that there are some things that seem to not be deterministic. In any event, we can approximate the behaviour of those around us, and we get better at it the better we know the people around us. Obviously, a deterministic system can be rational, but it is certainly not free.
There is a third option. A system that is partially deterministic and partially random. Computer based decision making systems are typically constructed according to this model (although the randomness is not true randomness, but that has no relevance here). Such a system can be very similar to a deterministic system, but even with perfect knowledge of the system it is not possible to fully predict its behaviour. However, when the system does not behave according to a "perfect" prediction, it does not do so out of rationality but out of chance. The behaviour in that instance will be chaotic. Therefore, these occasions are signs of free actions that do not fit the description of will, because they cancel out the effects of premeditation. This system therefore does not fit the idea of free will.
So my question to free will proponents is this: if none of the 3 types of systems above fit free will, then just what the heck IS free will? I've asked that question many times, and NOONE has been able to give me an answer.
So how can you argue for something that isn't even possible to define? How do you propose that free will can exist when the very concept of free will is so nebulous that noone knows what it is? I've heard people declare that it is none of the three options above (which is why I brought them up). But a definition must include a positive description - saying what free will isn't is not enough. We must know what it IS if you are to assert its existence.