The notion of free will pervades nearly every facet of society: religion, politics, responsibility, justice, morality and casual discourse. We frequently find ourselves appealing to some vague concept of the notion, whether we're actively ascribing moral responsibility (e.g., "he should be brought to justice") or making a morally indifferent judgment about another's choice (e.g., "I didn’t expect you to pick that one"). But for all its intuitive appeal and ubiquity, free will is one of the most hotly debated topics in contemporary philosophy. 

The free will debate, which concerns the extent to which humans have free will, largely revolves around determinism — the idea that only one future is possible given the laws and state of the universe. In essence, determinism is the proposition that everything which actually occurs (save, perhaps, quantum events) must actually occur, in the same way, a rock which tumbles off a cliff unhindered must fall downward. If determinism is true, it follows that human beings aren't free to alter the future: to do other than they do. 

For some, this aspect of determinism undermines free will. Philosophers who hold such a view are called incompatibilists, as they believe volition and determinism are incompatible. Antithetically, compatibilists believe that some form of free will is compatible with determinism, arguing that volition doesn't require a mutable future. They often accomplish this by qualifying free will as a conditional ability (e.g., despite the fact that I bought the brown shoes, and had to buy the brown shoes because the universe is deterministic, I was free to buy the black shoes if I could have bought the black shoes upon choosing to do so). Notice that this conditional analysis of the ability to do otherwise — to buy black shoes over brown — doesn't require determinism to be false. I don't need to be able to buy black shoes for the statement I could buy black shoes if I chose to do so to be true.   

While compatibilists universally argue for the intelligibility of deterministic freedom, incompatibilists are less cohesive. There are two opposing forms of incompatibilism: hard determinism and libertarianism. Hard determinists argue for determinism while repudiating free will. On the other hand, libertarians argue for free will while repudiating determinism. As such, libertarians argue for a more robust version of free will than compatibilists. They generally hold that free will requires two abilities, both of which they believe humans to have: the ability to actually do otherwise (e.g., despite the fact that I bought the brown shoes, I really could have bought the black ones) and the ability to be the sole cause of (some of) one's actions.

Though most philosophers now subscribe to some version of compatibilism (with libertarianism in second and hard determinism as a rather unpopular third), the free will debate is still very much alive. Indeed, it's likely that a stipulatory resolution to the dispute won't come anytime soon, if ever.