Let’s look at the following argument I saw on a different website:
1. If Adam has [sic] chosen otherwise, there would be no evil.
2. It was possible for Adam to choose otherwise.
3. There is a possible world without evil.
C. All evil is unnecessary.
As to logical validity, (3) follows but (C) comes out of nowhere. There is a hidden premise somewhere in here that needs to be brought out and so the argument holds no force as is since it’s not logically valid. However, it will be an interesting exercise to look at the argument.
First, (1) is not obvious for it seems clear that it could be the case that Adam chose otherwise in his first sin and yet sinned latter. Or, if we see the serpent as sinning then that would be another example. Moreover, Eve also sinned so there’s another counterexample. So (1) is not correct as is, so it would need to be reformulated.
As to (2), that’s not as obvious either. I suppose the argument wants to say something like the following:
(2.1) If Adam has libertarian free will, then it’s possible that he chooses otherwise.
(2.2) Adam has libertarian free will.
(2) Therefore, it’s possible that he chooses otherwise.
I personally do not find (2.1) very compelling myself. I tend to think that the Frankfurt counterexamples to the principle of alternative possibilities are sound and so (2.1) is not obvious as is.
But let’s say the argument is reformulated so that (3) follows and the argument is sound. So what? The problem here is that it is missing still a further premise. Namely:
(4) If a world is a possible world, then that world is a feasible world.
However, molinism denies (4) and so any problem is avoided.
All in all, therefore, it seems like a bit closer reading of Plantinga would clear up this argument right away. The free will defense still stands, then.