Libertarians hold that free action and moral responsibility are incompatible with determinism and that some human beings occasionally act freely and are morally responsible for some of what they do. Can libertarians who know both that they are right and that they are free make sincere promises? Peter van Inwagen, a libertarian, contends that they cannot—at least when they assume that should they do what they promise to do, they would do it freely.2 Probably, this strikes many readers as a surprising thesis for a libertarian to hold. In light of van Inwagen's holding it, the title of his essay—‘Free Will Remains a Mystery’—may seem unsurprising.

Although, as I will explain, van Inwagen's effort to motivate his contention about promising is problematic, an interesting challenge to libertarians that is focused on promise making can be motivated. In this essay, I will motivate a challenge of this kind, identify three ways libertarians may try to answer it, and develop one of the answers.

Van Inwagen's Predicament

As part of an argument against the theoretical utility of agent causation, van Inwagen asks his readers to imagine an indeterministic world in which he knows, perhaps because God told him, that there are ‘exactly two possible continuations of the present’: in one, he reveals a damaging fact about a friend to the press; in the other, he keeps silent about his friend. He also knows that ‘the objective, “ground-floor” probability of [his] “telling” is 0.43 and that the objective, “ground-floor” probability of [his] keeping silent is 0.57.’