I think that the problem is in definition (at least partly). Do you have free will? (rhetorical...) Yes? What does that mean in practical terms?
I understand the deterministic viewpoint of cogitation - in the sense that our minds appear to make decisions for us to do or not do something. Those decisions are the result of biochemical processes that should be deterministic. It's all about the chemical potentials of various neural pathways. Transmit a signal? It happens because of a flood of neurotransmitters from axon to dendrite. So in that sense, it all seems to be pre-determined. Except... that there are agonizing decisions that we all make, decisions that take inordinate amounts of time to finally pick a path and follow it. Sometimes that deterministic viewpoint isn't so well determined. If it were always fully determined, we would never hesitate at any choice.
If that is the case, let's talk about why we hesitate - and what mechanism is relevant here. Depending on how much you accept Transactional Analysis, you might accept that we are RARELY of one mind about anything - we are of THREE minds. These are the child self, adolescent self (a.k.a. parent self), and adult self. Freud might have called them the Id, the Ego, and the Super Ego.
Many psychologists believe that the three parts are EACH capable of contributing to a decision as though they are all independent - like, perhaps, a multi-processor computer where all THREE sections are churning away at something or another. Good thing it's an odd number of parts, 'cause if it were only two parts, we would have a lot of deadlocks. Whether they are independent or not, they are interdependent.
When you hear someone say "I'm of two minds on that topic" it might not be a joke or a turn of phrase. One of the reasons that people feel emotional pain over some sudden deep, dark family revelation is that they are experiencing cognitive dissonance as they try to process what they have just heard. That dissonance occurs when two different parts of the mind clash so strongly that you cannot immediately reconcile the conflicted feelings.
Case in point: My religious beliefs are well known. When I discuss atheism, many people can casually say "That's your choice" and move on. But when you find radicalized believers, their dissonance comes from the clash that their child self (keeper of instinctual behavior) and parent self (keeper of instilled/learned behavior) cannot reconcile the idea that a highly trusted person might have told them something that wasn't true. Maybe the child self still fears Hell. Maybe the parent self trusts what was taught from an early age. (Probably depends on each individual case.) If Mom and Dad taught you religion but then someone attacks the basis of your religious belief, your parent self CANNOT reconcile with the idea that Mom and Dad lied (or merely were wrong.) The child self feels the emotion of terror that a long-instilled belief might be wrong and thus causes the extreme reaction often seen in strongly confronted religious zealots.
What has this to do with free will? Have you ever heard of "chaos theory"? That theory talks about forces ALMOST in balance, so close to it that the result is highly sensitive to earlier factors and influences. The key phrase is "Sensitive Dependence on Initial Conditions" or SDIC. When you are in balance between the adult (rational) self and one of the other two selves, the factors that assist you in making that decision might be the color of the dress your new friend wore on the day you met even though the problem at hand might be purely financial. It might just be an association "out of the blue" that triggers you to be more or less favorable to some new thing. This is because our minds operate based on multiple contributing factors coming from different origins and even different times. What is it the bard said? "The past is prologue." Decisions you make today can (probably do) depend on all prior life experiences in different degrees.
So how does this tie into free will. The answer is that in practical terms, if chaos theory is at work, at the micromolecular scale you can have nearly matched conflicting processes. Who says that ALL neurons fire uniformly? (I surely don't!) Who says that your body's level of neurotransmitters is at full strength? Who says some other recent thought didn't momentarily deplete the neurotransmitters? Who says you don't have a very minor amyloid plaque build-up somewhere that blocks the flow of cogitation? The point is, all of those factors contribute in positive or negative ways to the flow of neural activity. If you have conflicted ideas and a chaos theory situation in the brain, the decision you make might depend on too many factors and incredibly minuscule factors such that your decision is not predictable. Your inner selves might disagree, requiring a resolution to occur first.
From a pragmatic viewpoint, therefore, if I cannot predict what you or I will do in a given situation, it is indistinguishable from free will. Further, having free will isn't precluded by all those biochemical processes. Based on a relevant discussion from author Randal Garret from the story Unwise Child, consider his description of the Yale Law of Animal Behavior: Given a set of lab animals taken from a breeding line that has run true for at least seventeen generations, placed in identical surroundings in a tightly controlled environment, and given the same initial stimuli with identical strength and timing, each animal will do as it Goddamned well pleases.
The (mis-)quote from that story reflects on the fact that the animal cannot do otherwise. Each animal does whatever feels right for it at the moment of the test. This is because of SDIC. What pleases animal X might be less pleasing to animal Y. If you want to call their reactions the result of biochemical determinism, fine - but then you have to explain why their reactions aren't identical. (And they will not be identical!) The EASIEST way is to say that frequently our actions are nearly in balance, awaiting only a reasonable external stimulus to trigger an action. BUT unless you can completely describe the biochemical state of the test subject at the exact moment of that stimulus, you cannot expect those animals to give you uniform results.
There, in the gaps between neurons and at the sub-microscopic level of chemical reactions that depend on recent repolarization of nerve endings, lies free will.
even death.
