Many (not all) advocates of libertarian free will endorse the Principle of Alternate Possibilities (PAP):
PAP: S is morally responsible for doing A only if S could have done otherwise.
PAP has come under continual fire ever since Harry Frankfurt’s seminal article in 1969, and many philosophers (including a number of leading libertarians) now accept that PAP is false. Leaving aside the philosophical arguments, however, it seems to me that any orthodox Christian ought to reject PAP on theological grounds.
Consider the following argument from the doctrine of Christ’s impeccability:
- Christ could not have sinned.
- Therefore, Christ could not have done otherwise than refrain from sinning.
- Christ was morally responsible when he refrained from sinning.
- If the ability to do otherwise is a necessary condition of moral responsibility, and Christ was morally responsible when he refrained from sinning, then Christ could have sinned.
- Therefore, the ability to do otherwise is not a necessary condition of moral responsibility (i.e., PAP is false).
The argument is deductively valid, so the advocate of PAP needs to reject one of the premises. It must come down to either 1 or 3, since 2 is just a restatement of 1, and 4 is a logical truth.
The defender of PAP might be most inclined to deny 1, but it must be recognized that there’s a very strong modal argument in favor of 1. If God is necessarily good then it’s logically impossible for God to sin; and if Christ is God Incarnate then it’s logically impossible for Christ to sin. Classical/Anselmian theism conjoined with Nicene/Chalcedonian Christology entails that Christ could not have sinned (in the strongest sense of “could not”). The logic of this argument is, one might say, impeccable.
So denying 1 isn’t an attractive option. But neither is denying 3, for several reasons. First, it entails that Christ’s sinless life was not morally praiseworthy, because moral responsibility is a necessary condition for moral praise. (What Christian would be comfortable saying that his life, despite all his failings, is more morally praiseworthy than Christ’s?)
Second, it seems to undermine the moral significance of Christ’s temptations. What does it matter that Christ was “one who in every respect has been tempted as we are, yet without sin” (Heb. 4:15) if Christ wasn’t morally responsible when he resisted temptation?
Third, it undermines the idea, widely held within the Christian tradition, that Christ merited our salvation by living a perfect life in obedience to the moral law. How could there be merit without moral responsibility?
I therefore conclude that anyone who holds to an orthodox Christology ought to reject PAP. Needless to say, this poses a problem for Arminians who want to wield PAP against their Calvinist brethren.
I very much doubt that I’m the first person to come up with this argument, but I don’t recall coming across it before.