Free will is a basic datum of experience, on which our mind bases itself in order to comprehend reality; to deny this is to deny the mind.
And those same who deny free will in theoretical debate accept it wholeheartedly on every other sphere of their lives; this complete adhesion is manifested through their actions and daily life, which is lived as if choices are being made.
Without free will, there ceases to be even the possibility of rationally grounding any human action.
To maintain in discussion what one does not truly believe to be the case is dishonesty with oneself.
The argument against the possibility of free will starts by denying the possibility of free will; it is circular, convincing only to those who already accept its conclusion.
Here it is, briefly stated:
All events are either deterministic or randomic.
Acts of choice are events.
If the mind (or mental acts) behaves in a deterministic or randomic way, either way its choices cannot be said to be free.
-Therefore, free will does not exist.
In accepting the first premise, one has already defined voluntary choice out of existence. One has already denied that there is a categorical difference between the falling of a rock or the movement of an electron and a rational, conscious action.
In reality, our mind cannot escape the clear evidence of experience: voluntary, along with randomic and mechanically caused, constitutes an unanalyzable category of reality, the denial of which leads to absurd conclusions, and to the blinding of our own intellects.
The materialist who denies free will behaves similarly to the metaphysician who denies mechanical causality (every event would be, to him, the manifestation of someone's intention). However, whereas the latter was only left with a goofy way of thinking about nature, the first reaches conclusions that should have, were he consistent, the most disastrous consequences to his and other people's lives.