Welcome back to "Doggerel," where I ramble on about words and phrases that are misused, abused, or just plain meaningless.
Free will is one of those things I find incredibly difficult to define precisely, even though I have a sense of what it means. On a casual level, it can be used to describe a trait possessed by sapient beings: They can make a wide variety of decisions about how they live their lives. That variety diminishes as you go down to simpler minded creatures, like ants, who are pretty much hardwired or close to it.
The trouble comes when you get down to the nitty gritty.
First, there's causality. Everything I do is affected by my genes, my experiences, my knowledge about my current situation, the stimuli I'm receiving by now, and even my medication. All of that affects the cognition behind my decision process. Someone who knows a lot about me could do a decent job of predicting my actions. In principle, there's no reason not to believe keeping track of all the particles in my corner of the universe could yield a near perfect prediction of my behavior.
That doesn't sit well with a lot of people. They don't like the idea of being a "slave" to external forces, instincts, drugs, or whatever. Some of us who have come to believe in determinism still get a twinge of discomfort every once in a while. The problem is what else is there? Adding in a "supernatural" component like a "soul" doesn't solve the problem. If it's not bound to the various influences in your life, why is your behavior still fairly predictable based on those influences? If it's just something that occasionally makes you act contrary to your nature, how's it any different than any seemingly random natural event?
People are complex things. It's far more likely unexpected or counter-intuitive behavior is a result of that complexity than an ethereal set of dice attached to everyone's brain.