From chapter four of God: A guide for the perplexed by Keith Ward (131-32), "The God of the Philosophers."

"Most philosophers, whether they believe in God or not, think that everything in the universe is caused. So if we knew the laws of physics or the will of God completely, we would see that things just have to be the way they are. There are no alternatives. But they also think that human beings are properly held responsible for their actions, at least sometimes, and therefore that they are sometimes free to do otherwise. So they have the problem of seeing how somebody can be free to do otherwise, when there is no alternative to what he or she does.

"Most philosophers, and not just Augustine, have thought you have to believe both of these things, that there are no alternatives to what [132] happens, and that people are sometimes free to do otherwise. This is called compatibilism. Augustine believed it. Aquinas believed it. Calvin believed it. It means that causal determination by the laws of physics or by the will of God is compatible with human freedom and responsibility.

"The opposite of compatibilism is libertarianism. This is not, as some of my American colleagues think, the right to own a sub-machine gun or to live on Venice Beach with a preferred other of your choice. It is the view that a truly free act cannot be wholly determined by any prior state or being or law of nature. A free act might be determined in many ways, but part of it is due to a really new decision of the agent, which is unpredictable in principle from any law. Even God could not predict it, since part of the act is wholly in the power of the agent. Most compatibilists pretend that they do not understand what this means. They say that such an act would simply be random, and therefore irrational. Whatever you think about this, it is a fact that most philosophers throughout history have been compatibilists, and have thought that predictability or determinism does not detract from any freedom that is worth having."