This is the fortieth post in my Theories of Knowledge and Reality series. Follow the link for more on the series and for links to other entries as they appear. In the last post, I looked at some of the arguments that have been offered in favor of taking ourselves to have free will. In this post, I'd like to distinguish between the different views one can take on the two main issues at stake in this debate, with some attention to which combinations of these views are coherent.
I've already given a definition of determinism, and its denial is indeterminism. If the laws of nature and any state of the world guarantee any later state of the world, then determinism is true. Otherwise, indeterminism is true. The question that immediately arises is how we should connect that issue up with the question of free will. To settle that, we first need an account of what free will is, and then we can answer the question of whether we could be free in a world that is deterministic. In a coinage that, unusual for philosophy, actually reflects ordinary, contemporary English, the view that freedom and determinism are compatible is called compatibilism. Thus some views of freedom will be compatibilist accounts of freedom, while others will be incompatibilist.
Incompatibilism splits into two main camps, libertarianism and hard determinism. Both affirm that we can't have free will if determinism is true, but each chooses a different horn of the dilemma. Libertarians (not to be confused with political libertarians) affirm human freedom and deny determinism, while hard determinists accept determinism and deny free will. The standard libertarian account of freedom includes at least one thesis, sometimes called the Principle of Alternative Possibilities (PAP). PAP says that it is possible to do otherwise than what we actually do. Whatever we did do, we could have done something else. We'll look at some variations in what else a libertarian might say, but most libertarians do accept this component. For now it will suffice to see that if determinism is true, then only one future seems possible, since the laws of nature together with the past will guarantee the future, exactly one future, without other options for what that future will be. So if determinism is true, then we seem not to have alternative options. Thus freedom requires indeterminism if libertarianism is correct.