And of course Free Will is the crux of the argument against the Problem of Evil.
What about this:
Avoidability condition on freedom: I do something freely iff I could have acted otherwise.
"In whatever manner man is considered, he is connected to universal nature, and submitted to the necessary and immutable laws that she imposes on all the beings she contains, according to their particular essences...Man's life is a line that nature commands him to describe upon the surface of the earth, without his ever being able to swerve from it, even for an instant..."
This law of causality Holbach speaks of is more or less what is known today as:
Determinism: Whatever happens is determined by prior events.
Human choices and actions are no exception. Whatever you decide to do, your decision was inevitable given prior conditions. Granted the decision feels free but this is an illusion. (Such illusions are not unknown. Post-hypnotic suggestion: subject on hands and knees, I was wondering about the quality of the tile etc.) And if the decision is not free -- if you couldn’t have chosen differently -- then neither is the resulting action. For you couldn’t have acted any differently than in fact you did.
Hard determinism is the view that first, determinism is true, and second, this shows that freedom is an illusion. Here is the basic argument.
(1) Whatever happens is determined by prior events. (Determinism)
(2) I act freely only if I'm able to act otherwise. (Avoidability Condition)
(3) If my action is determined, I'm unable to act otherwise.
(4) Therefore, I don’t ever act freely. (1,2,3).
Seeing this argument, you might argue that the best strategy is simply to deny determinism. But does that solve the problem? Is indeterminism any more compatible with freewill?
Indeterminism: Some events are not determined by prior events.
Consider the freewill dilemma (see also p. 387-8 of Reason and Responsibility):
(1) If determinism is true, we can never do other than what we do; so we are not free.
(2) If indeterminism is true, then some events--possibly some actions--are random; but if they are random, we are not their authors. So we are not free.
(3) Either determinism or indeterminism is true.
(4) Therefore, we never act freely.
It appears that denying determinism is no help in preserving for us some space for genuine freedom.
I'm not saying I agree with it. Determinism is obviously very controversial, but it has a point. Being morally inclined myself I'd like to believe in Free Will. Whether it exists or not is an entirely impossible question to answer. Of course, everything in philosophy is.