If you think back to Descartes and a lot of the philosophy about determinism and free will, they had quite a naive understanding of systems and determinism beca

If you think back to Descartes and a lot of the philosophy about determinism and free will, they had quite a naive understanding of systems and determinism because their mental model of a complex system was a clockwork watch or a steam engine with lots of gears. Because their argument was, that if the universe is deterministic everything seems a bit scary because whats going to happen is already 'known' and so we have no free will. But now we have a better understanding of how mind bogglingly complex deterministic systems can be - broadly this is what Chaos Theory is all about - e.g. sensitive dependence on initial conditions - you can model how a magnetic pendulum moves between three magnets, but the tinyest infinitessimal change in the starting position makes a big difference to the eventual path. And we know that through various fundamental forces, every subatomic particle interacts with every other one in its light cone. And so if we assume that the universe is deterministic, we can see it would be impossible to build a computer to exploit that determinism and 'calculate ahead' to see what state the universe would be in at some future point. Two main reasons - (a) the computer would inevitably use more atoms than the atoms it was trying to simulate and (b) sensitive dependence on initial conditions. And so even in a deterministic universe, the future is literally unknowable. So you can think of the universe as a computer that is calculating its own future, one picosecond at a time. We can't jump ahead. (we can, of course, use simplified models to give us good guesses for specific short term situations) That means determinism is nothing to worry about, in terms of free will. (and anyway, how would the opposite - randomness - be any better for free will?) (so what is Free Will? Its not really a concept that makes any sense - "I chose to do X but in completely identical conditions I could have been free to choose Y" - how do you even test that? But if you want a colloquial definition: Broadly I'd say we have something that you might choose to call "free will" because what we do next is (usually) proximately determined by processes inside our skulls. And yes, if you want to trace the state of our brains back in time eventually all of it originated from outside our skulls, memories etc, but lets set all that historical complexity aside. What we do next is broadly determined by processes within us. As opposed to a rock rolling down a hill which is just subject to external forces. I think thats the only thing Free Will can really mean)