Some topics should be left alone. Unless it is our profession to spend many hours pondering such things, our personal quest for truth can become vain and slothful.
Anyway, here’s a concise explanation by St. Thomas Aquinas:
Chapter 10: That divine predestination does not impose necessity on human acts
Last of all we come to the question whether, because of divine ordination or predestination, human acts become necessary. This question requires caution so as to defend the truth and avoid falsity or error.
It is erroneous to say that human acts and events escape God’s fore-knowledge and ordination. It is no less erroneous to say that God’s fore-knowledge and ordination imposes necessity on human acts; otherwise free will would be removed, as well as the value of taking counsel, the usefulness of laws, the care to do what is right and the justice of rewards and punishments.
We must observe that God knows things differently from man. Man is subject to time and therefore knows things temporally, seeing some things as present, recalling others as past, and fore-seeing others as future. But God is above the passage of time, and his existence is eternal. So his knowledge is not temporal, but eternal. Eternity is compared to time as something indivisible to what is continuous. Thus in time there is a difference of successive parts according to before and after, but eternity has no before and after, because eternal things are free from any change.
Thus eternity is totally at once, just as a point lacks parts that are distinct in location. For a point can be compared to a line in two ways: first as included in the line, whether at the beginning, middle or end, secondly as existing outside a line. A point within a line cannot be present to all the parts of the line, but in different parts of the line different points must be designated. But a point outside the line can view all parts of the line equally, as in a circle, whose central point is indivisible and faces all the parts of the circumference and all of them are somehow present to it, although not to one another.
An instant, which is a limit of time, is comparable to the point included in a line. It is not present to all parts of time, but in different parts of time different instances are designated. Eternity is something like the point outside a line, like the centre of a circle. Since it is simple and indivisible, it comprehends the whole passage of time and each part of time is equally present to it, although one part of time follows another.
Thus God, who looks at everything from the high point of eternity, views as present the whole passage of time and everything that is done in time. Therefore, when I see Socrates sitting, my knowledge is infallible and certain, but no necessity is imposed on Socrates to be seated. Thus God, seeing everything that is past, future or present to us as present to himself, knows all this infallibly and certainly, yet without imposing on contingent things any necessity of existing.
This comparison can be accepted, if we compare the passage of time to travel over a road. If someone is on a road over which many people pass, he sees those who are just ahead of him, but cannot certainly know those who come after him. But if someone stands in a high place where he can see the whole road, he sees at once all who are moving on the road. Thus man, who is in time, cannot see the whole course of time at once, but only thinks that just in front of him, namely the present, and a few things of the past, but he cannot know future things for certain. But God, from the high point of his eternity sees with certitude and as present all that is done through the whole course of time, without imposing necessity on contingent things.
Just as God’s knowledge does not impose necessity on contingent things, neither does his ordination, by which he providentially orders the universe. For he orders things the way he acts on things; his ordination does not violate but brings to effect by his power what he planned in his Wisdom.
As for the action of God’s power, we should observe that he acts in everything and moves each single thing to its actions according to the manner proper to each thing, so that some things, by divine motion, act from necessity, as the motion of heavenly bodies [according to ancient cosmology], while others contingently, which sometimes fail in their proper action because of their corruptibility. A tree, for example, sometimes is impeded from producing fruit and an animal from generating offspring. Thus Divine Wisdom orders things so that they happen after the manner of their proper causes. In the case of man, it is natural for him to act freely, not forced, because rational powers can turn in opposite directions. Thus God orders human actions in a way that these actions are not subject to necessity, but come from free will.
