What is it about free will that makes it such a difficult problem? One obvious answer is its complexity. Although philosophers refer to the problem of free will, it is in fact made up of a number of interrelated problems: Do we have free will? Why do we think that we have free will? What affects or limits our free will? Do such affecting or limiting factors apply equally to everyone? How is free will related to moral responsibility? Is free will compatible with determinism (the thesis that there is only one physically possible future or only one physically possible outcome following a series of events)? This last question raises what is for many philosophers the core problem of free will. Over the centuries the determining agent has varied (fate, God, the laws of nature or logic, our heredity and environment, and social conditioning, to name the most obvious contenders), but the overall fear has remained the same: are we determined to make the decisions that we make, and if we are, in what meaningful or valuable sense, if any, are our decisions free?

Since at least the seventeenth century, the free will debate has been dominated by three main groups, defined by their attitude to determinism. Those who believe that free will is incompatible with determinism are known, with admirable clarity, as incompatibilists. They can be divided into two opposing groups. Those incompatibilists who reject determinism because they believe that we have free will are known as libertarians.