A few years ago, I was doing my pre-race day run and, for no apparent reason, my left leg began to hurt. I made my way home, estimating the odds of a recovery by the next 
day. On the morning of the race, my leg felt better and my short pre-race run went well. Just before the start, I was optimistic: it seemed my leg would be fine. Then the race started. Then the pain started.
I hobbled forward and “accelerated” to an 8:30 per minute mile (the downside of a GPS watch is that I cannot lie to myself). The beast of pain grew strong and tore at my will. Behind that armor, my fear and doubt hid—urging me to drop out with whispered pleas. At that moment of weakness, I considered doing the unthinkable: hobbling to the curb and leaving the race.
From the inside this seemed a paradigm example of freedom of the will: I could elect to push through the pain, or I could take the curb. It was all up to me. While I was once pulled from a race because of injuries, at that time I had never left one by choice—and I decided that this would not be my first. I kept going and the pain got worse.
At this point in the race, I considered that my pride was pushing me to destruction or at least a fall. Fortunately, decades of running had trained me in pain assessment: like most veteran runners I am good at distinguishing between what merely hurts and what is causing significant damage. Carefully considering the nature of the pain and the condition of my leg, I judged that it was mere pain. While I could have decided to stop, I decided to keep going. I did, however, grab as many of the high caffeine GU packs as I could—I figured that being wired would help with pain management.
Aided by the psychological boost of my self-medication (and commentary from friends about my unusually slow pace), I chose to speed up. By the time I reached mile 5 my leg had gone comfortably numb and I increased my speed, steadily catching and passing people. Seven miles went by and then I caught up with a former student. He yelled “I can’t let you pass me Dr. L!” and went into a sprint. I decided to chase after him, believing that I could still hobble a mile even if I was left with only one working leg. Fortunately, the leg held up better than my student—I got past him, then several more people, then crossed the finish line running a not too bad 1:36 half-marathon. My leg remained attached, thus vindicating my choice. I then chose to stuff pizza into my pizza port—pausing only to cheer on people and pick up my age group award.
As the above narrative indicates, my view is that I was considering my options, assessing information from my body and deciding what to do. That is, I had cast myself as having what we philosophers like to call free will. From the inside, that is what it seems like. Maybe.
Of course, it would presumably seem the same way from the inside if I lacked free will. Spinoza, for example, claims that if a stone were conscious and hurled through the air, it would think it was free to choose to move and land where it does. As Spinoza saw it, people think they are free because they are “conscious of their own actions, and ignorant of the causes by which those actions are determined.” As such, on Spinoza’s view my “decisions” were not actual decisions. That is, I could not have chosen otherwise—like the stone, I merely did what I did and, in my ignorance, believed that I had decided my course.
Hobbes takes a somewhat similar view. What I would regard as a decision making process of assessing the pain and then picking my action, he would regard as a competition between two competing forces within the mechanisms of my brain. One force would be pulling towards stopping, the other towards going. Since the forces were closely matched for a moment, it felt as if I was deliberating. But the matter was determined: the go force was stronger and the outcome was set.
While current science would not bring in Spinoza’s God and would be more complicated than Hobbe’s view of the body, the basic idea would remain the same: the apparent decision making would be best explained by the working of the “neuromachinery” that is me—no choice, merely the workings of a purely mechanical (in the broad sense) organic machine. Naturally, many would throw in some quantum talk, but randomness does not provide any more freedom than strict determinism. Rolling dice does not make one free.
While I think that I am free and that I was making choices in the race, I have no way to prove that. At best, all that could be shown was that my “neuromachinery” was working normally and without unusual influence—no tumors, drugs or damage impeding the way it “should” work. Of course, some might take my behavior as clear evidence that there was something wrong, but they would be wrong.
Kant seems to have gotten it quite right: science can never prove that we have free will, but we certainly do want it. And pizza.