Let's take a toothpick for example. A scientist can measure the chemical compound of the toothpick, the environment that the toothpick is in, the forces that will act upon the toothpick, etc., and the scientist can calculate how long it will take the toothpick to decompose.

Now take that a little further. If you could somehow measure every molecule, every atom, every force in a given environment and calculate how they will respond and react to eachother, then you could predict how that enivironment and everything within it will have changed in a given timeframe.

If you could measure every atom, every force, every reaction, in the universe then calculate the outcome, then you could predict anything about the future- it would just be a giant math problem. Now the question is, Is a human being's mind just chemicals and electrical signals and the-like acting upon one another? If so, then it could all be calculated along wth all the other atoms and forces in the universe in this math problem. And so, we find that our destiny is set. The future, every action, is the result of all of these forces acting upon one another.

Or does a human have free will, in control of his own fate?