Regardless of what free will ultimately is, and regardless of whether human beings – or anything else for that matter – has this property, it is undeniable that we are under the strong impression that we are free, and that our choices are our own to make.
For this reason, I believe that the most important thing in a discussion or an essay about free will is to determine what, exactly, it is that we are feeling. By identifying the probable mechanism(s) behind our impression of freedom, we can more easily determine whether that impression is related to any kind of hard reality.
What produces the impression of free will
I would say that this is relatively simple to understand, although I cannot vouch for the biology behind it. The gist of the idea is that your “self” corresponds to the model you have of yourself – whenever you think of your own “self”, although you might refer to your physical body and everything it contains, you can only do so through a mental model. Since this mental model is necessarily much simpler and cruder than the real thing, and that it is used to inform your decision process, it is not (cannot be) fully capable of predicting your behavior. Now, what happens here is that we all conflate the mental model we have of ourselves with the physical system it is supposed to model. The fact that the model is typically invariant to the actual decisions that are made, coupled with the observation that decisions are indeed made, leads to an unmistakable impression of free will.
In other words, whenever you do introspection – whenever you say “I” – you conjure a mental model of yourself, which you manipulate and combine with other concepts. But even though some slight perturbations in your brain might lead you to choose to wear shoes over sandals or vice versa, that does not really make any difference as to your conception of self. Typically, your conception of “self” cannot tell apart a brain that chooses to wear shoes from a brain that chooses to wear sandals. Even though they have different behavior, as far as your self-image is concerned, both of them are “you”. Because of this, you simply assume that you can do either action.
It might help to consider that all thoughts operate on mental objects, which are mere models of real objects. When we give an attribute to something, we are only tagging the actual object through the mental model we have of it (after all, even if we could acquire it, it is not like we can store all the molecular information of anything sizable in our brains). So when you say “this dog barks a lot”, you are saying “I model the object at this location as a dog that barks a lot, and you should model it the same way”. Similarly, when you say “I might ask her out”, you are saying “I model myself as a human that might ask this girl out, but not certainly, and you may use this information to improve the model you have of me”. In this context, free will is fairly easy to understand: the mental model we have of a human being is non-deterministic. It does not really matter whether an actual, physical human is deterministic or not because we apply the concept to the mental model.
Except, of course, that if we know that the universe is deterministic, we are compelled to enrich our mental models with that information. And that is where problems happen: indeed, the near totality of our mental models are non-deterministic. That is not necessarily a big deal as long as you realize that mental models are not the same as reality, but conflating both is so convenient in practice that we do it all the time (heck, many people are utterly incapable of grasping the difference between language and reality). Long story short, the idea that the universe is deterministic causes huge cognitive dissonance because it clashes with natural thought mechanisms. Different people cope with this in various ways. Personally, understanding why there is a cognitive dissonance suffices to dispel it in my own mind.
So what about free will?
To me, the impression of free will makes perfect sense – any entity capable of introspection, in so far that they model themselves in an incomplete fashion, would have an impression of free will. That is simply, as I said, because many different brains map to the same self-model, so the self-model M could be said to have the choice to do X if there exists a brain that does X and is modeled by M. Any action that does not contradict the model you have of yourself is an action you will think you can do. I do not see the impression of free will as necessarily resulting from a special or complex phenomenon.
Now, I could define “free will” as exactly corresponding to the conditions I believe lead to an “impression of free will”: “an entity has free will if it makes use of an incomplete model of itself as part of its decision process, and conflates that model with the physical entity it refers to” (note: that definition is not necessarily meant to be exhaustive). Indeed, I would say that all such entities would feel like they have free will, so it is not a huge stretch to outright define it as such (do note that I am usually rather liberal with definitions – I don’t care what a word means, as long as the definition is clearly stated).
I reckon that most people would not be satisfied with conflating free will with the impression of free will. That is, they would contend that everything I have said supports the idea that free will does not really exist, and that it is an illusion. I must say that I am perfectly fine with this interpretation (again, I am not the kind of person who bickers about definitions – clarity is all that matters to me).
Ultimately, though, if “free will” is a coherent concept, then this must mean there is a formal way to tell apart things that have free will from things that do not. For as long as a clear, formal test for free will is not devised and agreed upon, I must confess I have no idea what the debate is about. Some people view non-determinism as a sine qua non condition to free will, whereas compatibilism contends that determinism and free will are compatible. Personally, I am of the opinion that whether free will is compatible with determinism or not is a matter of definition, and I frankly could not care less which way it goes. All I know is that if you can’t agree on something this fundamental, that’s because you are talking about different things.
We all have the same impression, the same subjective perception that we have a freedom of choice. However, it is quite clear to me that different people have different ideas and different expectations as to what free will represents. The whole free will debate is about determining whether the impression we have conforms to our expectations about what causes it. Alas, there is a fundamental divide in the expectations of at least two groups of people: those that believe determinism precludes free will, and compatibilism. Don’t even try to determine which side is “right”: the difference lies solely in expectations*. For this reason, there is no unified concept of free will and there probably never will be.
Long story short, I believe I have presented a nice and coherent explanation as to why we feel that we have free will – I would be quite interested to know how close I am to being correct. On the other hand, I do not care whether free will is an “illusion” or not, and believe that all debates of that nature are sterile. All arguments about free will are grounded in semantics, not in reality.
* Roughly speaking, you can understand “X freely chose to do Y” as either meaning that it is possible that X would not have done Y – or you can understand it as the weaker proposition that the fact that X did Y can be traced back to factors that are mostly internal to X. The first definition can be seen as more intuitive, but naive, incomplete and ill-defined. The second can be seen as better fleshed out, but less intuitive, self-serving and (again) ill-defined (what is “internal”? what is “external”?). To be honest, both views are defensible, in the sense that someone on the fence can be compelled to go either way. I don’t think either side will ever win, and if one does, I don’t care which one.