> Arguably, if you knew the exact system date and all other software installed, you could predict exactly how your program would behave. First of all, there is

> Arguably, if you knew the exact system date and all other software installed, you could predict exactly how your program would behave. First of all, there is a difference between what you can predict about the system and what you want the system to be. Since even on nondeterministic system you can build a deterministic one, it doesn't matter what you can or cannot predict about the underlying system. If you want a deterministic system, you can simply ignore the unpredictable parts by averaging over them. Although, if you know the other software, you could predict the system. But normal application, unless it has root privileges or there are privilege escalation loopholes, cannot inspect the whole system and cannot see all the processes that go on. Therefore, deterministic modelling of the system from a perspective of one application is impossible. But there is actually a simpler example. Just take an application using the pseudorandom number generator. You can't know if there are other processes using the generator too, so once you use pseudorandom generator, your application becomes non-deterministic. If on the other hand the application wants to be deterministic, all it has to do is not use system-wide generator of randomness, regardless of whether the generator is actually deterministic or not. The point is, for free will to exist, it doesn't really matter if the universe is deterministic. Just as it doesn't matter in D&D game if the result is determined by dice rolls or a pseudo-random number generator with a known starting seed. People cannot calculate the latter in their heads, so the difference won't affect their reasoning (these things are conditionally dependent on the observed reality); that's all that matters.