Note: the full text of the article below may be found at the following site:
http://www.sfu.ca/philosophy/swartz/freewill2.htm
Epistemic Determinism (or, The Problem of Foreknowledge)
The following is the standard argument for Epistemic Determinism. It alleges to show that foreknowledge is incompatible with free will.
If x knows that you are going to do (some action) A, then you must do A. But if you must do A, then you have no choice in the matter. Thus if x knows (beforehand) what you are going to do, then you have no free choice. Put another way: foreknowledge is incompatible with free will.
We will call this version of the Problem of Foreknowledge, the 'secular' version.
Often the argument is presented in its 'religious' version. In this second version, what is of primary interest to us is the relationship between omniscience and free will. For our purposes, I/we are neither assuming that God does exist nor assuming that God does not exist. We are simply examining the logical relationship between two concepts. (The puzzle occurs whether one is a theist or an atheist: Does foreknowledge preclude free will?)
God is omniscient, i.e. God knows everything (that is true) about the past, the present, and the future. In addition (it has been claimed), God has given human beings free will so that human beings can choose between good and evil.
But if God knows beforehand what you are going to choose, then you must choose what God knows you are going to choose. If you must choose what God knows you are going to choose, then you are not truly choosing; you may deliberate, but eventually you are going to choose exactly as God knew you would. There is only one possible upshot of your deliberating.
Thus if God has foreknowledge, then you do not have free will; or, equivalently, if you have free will, then God does not have foreknowledge.
The religious version of the argument for Epistemic Determinism is more than just a theoretical curiosity. A number of Christians have accepted the argument, and in doing so, have proceeded to live their lives in a different manner than many others, including the majority of Christians. Philosophical arguments can, and sometimes do, have profound effects on persons' lives. When they do, we owe it to ourselves to assure ourselves that the arguments are genuinely sound.
Both versions of the argument -- the secular and the religious -- are valid (i.e. the premises logically guarantee the conclusion). But are the premises true? As we might suspect, they are not. But what, then, is the error? Where does the falsity lie? To answer these latter questions, we will need to make a detour to examine some logical concepts, those having to do with possibility, actuality, and necessity.