I am not sure why the 'outside force' is required for free will. What characteristic does that provide that is needed for free will? If the physical world is n

I am not sure why the 'outside force' is required for free will. What characteristic does that provide that is needed for free will? If the physical world is non-deterministic, then there is nothing stopping a physical thing from having free will. If an 'outside force' can exert free will, why can't a physical thing?