If your view is that Brownian Motion is not deterministic then does that not contradict the idea 'that cause and effect holds in a closed system' as any sort of absolute?
The subject was free will. I said "determined" in contrast to free will. So, if you've got Brownian Motion (or, worse, quantum uncertainty), then you've got some randomness in there, and it's not a purely deterministic system. But that doesn't give you any more free will; it just means that the machine that determines your choices has a random number generator as one of the inputs. You still don't get to choose, because you don't control the randomness.