[Yasa]

1. Free will:

One has free will when his actions/choices are contingent. That is, the person can do, or not do something. He can choose something or not choose something. Give a certain decision, it is perfectly reasonable that he could have decided otherwise. If it is not true that the person could have chosen otherwise, then the person does not have free will.

2. Omniscience:

A god who is omniscient knows all, including the future, knows every action, decision, etc. that you will make.

3. If such a god's knowledge of the future is not false, then the actions foreseen for you must indeed happen and be unchangeable. In other words, they are not contingent and you are not free to choose otherwise.

4. Therefore, either a god cannot be omniscient and we have free will, or we do not have free will and a god could be omniscient.


Expansion on premise 3:

It is clear that if a being could see the future actions of everyone, then in order for such a being's knowledge of the future to be accurate, those actions must necessarily happen. They cannot happen otherwise, or else such a being's knowledge would indeed be false and thus we would not consider that being to have omniscience. However, should every action necessarily happen in a certain way, those actions are not contingent--they cannot happen otherwise. This implies that free will is merely an illusion.