As far as I can tell, everything is predetermined. Although I wouldn't call it destiny or fate. Those words have connotations that I do not agree with. Rather, everything is predetermined simply because everything is a series of inconceivably complex reactions.
As far as I know, all things in the universe act in accordance with observable and consistent rules. If one had enough information, the knowledge needed to apply it and enough time to do all the math, then one would be able to perfectly predict the future.
"What about human action and free will?"
This too I believe to be effectively pre-determined, but it's a bit harder to explain. I shall try anyway.
Imagine you are faced with a decision. Any decision. Great or small. For the sake of simplicity, we'll use a small example. Say... which shoes you are going to wear today.
You would consider your options, then make a decision based on a huge number of factors. It's cold today, so the sandals are out. Those new boots are nice, but you haven't worn them in yet, they still give you blisters and you just can't be fucked with that today. You could wear those new yellow sneakers, but yesterday your friend said they look ugly as hell. "Like you have giant marshmallows stuck to your feet", I believe was the exact phrase. Now, much as you hate to admit it, you feel self conscious about wearing them. So that leaves your old black sneakers. They're a bit battered, but they're warm, comfy and don't bear the slightest resemblance to giant marshmallows. So black sneakers it is. Of course, in real life this decision would probably take the merest fraction of a second and would involve even more factors, but I think you get the point. Your free will has been exercised and you have successfully decided which shoes to wear. Congratulations!
Now, imagine if that scenario and that decision were to be perfectly recreated and then made to occur a second time. And I do mean perfectly recreated. Same weather, same shoes, same blisters, same marshmallow comment. You're in the same mood, you've had the same amount of sleep. Everything is the same. Hell, you've even had the memory of the previous occasion neatly removed from your head (because the original decision didn't involve a memory of having made an identical decision in the past, neither can the second in order for it to be a perfect recreation.) Even your age has been miraculously reversed. For all intents and purposes, time has been rewound and you are once again choosing which shoes to wear.
My question is, would you make the same choice you did this first time?
I believe that you would. Decisions, the exercising of free will, are based on the circumstances that surround them (or, if you wish to be precise, they are based on the decider's understanding of the circumstances). So if the circumstances are completely known the outcome of the decision can also be known.
If you agree with this, then that means that free will is pre-determined. It is predictable in the same way that two billiard balls bouncing off one another at a particular angle are predictable.
This is why I believe that everything, including out much vaunted free will, is predetermined, even though it doesn't feel that way.
