Do we have free will?

Suppose for a moment that everything that happens in the universe is predetermined. In this universe, the reaction of every particle of matter in the entire universe could in theory be calculated. That would mean that every thought you have would be a product of your mental state at the moment plus any external stimuli. This universe would be like a row of dominoes - the first domino topples the next, which topples the next and so on.

However, we humans don't have the capacity to accurately calculate such interactions between even two sub-atomic particles, much less between all particles in the universe. More importantly, we don't have the ability to see the events in our world as the consequence of these complex interactions. So even if the universe were in fact predetermined, it would still appear to us that our choices were the result of free will -- even if we were were predetermined to make those choices.

If the universe is really non-deterministic, if the future is not predictable even in theory, then we might actually have free will. If we actually had free will, then we would also have the feeling of free will. On the other hand, if the universe is in fact deterministic (predictable), we still have the feeling (the illusion) of free will. If we have the feeling of free will whether we have it or not, then we can't use the feeling of free will to tell us if we live in a deterministic or a non-deterministic universe.