I know it got messy when the NPs tried to discuss this one, but hopefully I'm gonna keep this focused to a specific point. Here we go...
I feel like I make decisions all the time. From whether or not to scratch an itch, to whether or not to kill my noisy neighbour. But here is the problem, in the form of a thought experiment:
Two identical people in two identical universes in two identical situations face the same identical choice.
They each have the same upbringing, knowledge, maturity, brain chemistry, personal preferences, everything is the same.
Can their decisions differ?
If the answer is no, then a persons choice is not free will but simply the result of the state of the collection of atoms that represents that person.
If the answer is yes, then what can account for this difference without resorting to non-naturalistic causes such as the soul?