How Does God Know Contingent Things?
Aquinas’s solution is mysterious:
Hence, whoever knows a contingent effect in its cause only, has merely a conjectural knowledge of it. Now God knows all contingent things not only as they are in their causes, but also as each one of them is actually in itself. And although contingent things become actual successively, nevertheless God knows contingent things not successively, as they are in their own being, as we do but simultaneously. The reason is because His knowledge is measured by eternity, as is also His being; and eternity being simultaneously whole comprises all time, as said above… Hence all things that are in time are present to God from eternity, not only because He has the types of things present within Him, as some say; but because His glance is carried from eternity over all things as they are in their presentiality. (ST, I, 14, 13)
The last sentence can be meaningfully interpreted in the sense that causes do not fully determine their effects. But it seems that they have to. God must see agent A in circumstances C, and from that deduce that the agent will do either x or y or whatever. Given indeterminism, if God sees A do x, how does He know that in the actual world A will in fact do x? What He sees is a possible world, one of many. If there is indetermination, then it is unclear which possible world will ensue when it comes to pass. How does God know that when it’s time for A really to choose, A will choose x? If we want to invoke pure randomness and God disposing of creaturely proposals to do x or y, or of an atom’s “preference” to decay or not, or of a virtual particle to pop into existence or fail to do so, then the set of causes which fully determine the future includes God as the first cause. But there has to be determination; otherwise there is no way for God to foreknow.
Suppose that I’m at the top of a tower, and I drop a heavy ball. I foreknow (with a high degree of probability) that it will fall to the ground. How do I foreknow? By seeing the effect in the cause. A certain aspect of the future is necessarily determined: the effect must occur given the entirety of the cause and the passage of time. God foreknows in the same manner. Foreknowledge is possible, because even though humans can do numerous things, they desire to do something definite at any given moment. They literally do not will to do other things, though they can do them. If God knows the desire, and if in addition the agent knows how to satisfy it, then He can predict human actions.