This is an issue that confuses me quite a bit by defining a term and proving a conclusion with that definition of the term. This does not seem appropriate for logical and empirical reasoning. It seems as though that we all understand the concept of free will, but we all have different takes on what would prove and disprove the existence of the concept. This makes it quite difficult to argue about, especially in that if you agree to go by somebody's definition of free will, and their definition would logically conclude that free will by their definition does not exist, then you must also agree with them that free will does not exist. Where this gets tricky is that by finding yourself to their definition for arguments sake, it makes it much more difficult to challenge their definition at a later point.
An easy way around this is to challenge their implication about free will and how their conclusion necessarily implies the lack of free will or the free will. I personally am having much difficulty coming up with some kind of argument for or against free will as I am unsure what would be a proof or disproof of it.
It seems possible to me that this is a concept that doesn't exist in reality. I am currently unable as of now to make a strong enough argument in support of this, but I am quite open to the concept of free will being an unreal concept, and that it cannot be proven or disproved through physical tests. If you have a hypothesis that rocks fall down, there is a means to testing this hypothesis as it directly correlates with physical reality. It would make little sense to conceptualize and to attempt to prove that rocks fall down with any conceptual test. Only physical means are applicable for testing a physical theory, meaning that the way to test if rocks fall down is to drop a rock and see if falls down. If you have a theory about this field at mathematics, logic, philosophy, or economics, you do not test this theory through physical means. 2+2 equals four is not proven through physical means, but rather means that comply with unreal concepts. This is not to say that a mathematical proof cannot be demonstrated through physical means, such as schoolkids counting on their fingers, but rather that the act of counting the fingers is completely irrelevant to the proof of the concept.
What I am trying to say is that I am unsure that a physical proof would make sense in the area of free will. Most arguments in this area tend to rely on physical proofs, such as: I'm currently choosing to do some action, therefor I have free will; the deterministic viewpoint of everything being predictable and there only being one possible timeline.
I will write more on the subject later.