Although I've already covered this topic elsewhere on my blog, after having attempted to explain this to a particularly stupid pair of theists for over two hours this afternoon, I feel compelled to explain it again, hopefully more clearly.
The topic is free will and god's omniscience being incompatible.
First a recap of what omniscience actually means;
1. universal or infinite knowledge.
2. the state of being all-knowing.
This, by definition, means that god knows everything, including what decisions we will make before we make them. This poses a big problem for free will, because it cannot exist.
Let's take an example;
- Tomorrow you will be offered a choice between a red and blue pill.
- God knows beforehand that you will pick the red pill.
- You cannot pick the blue pill without proving god wrong by contradicting his foreknowledge.
Unless you can pick the opposite of what god knows you will pick you can only ever have the illusion of free will. It may appear you had a free choice but to all intents and purposes the other pill may as well not be there, you could never choose it.
One of the more ridiculous defences I've seen is that this scenario is false because god doesn't influence our decision. It's logically irrelevant whether he does or not, the result is the same – if he knows you'll pick red you cannot pick blue whether he moves your hand or not, doing so would still make his foreknowledge wrong.
Another argument that defenders of freewill and omniscience compatibility will use is that god knows the outcome of all possible choices. At first glance this appears reasonable, but it's just as fallacious when examined logically.
This is a more difficult scenario to explain but bear with me, I'll do my best.
Imagine your life as a tree. It starts at the base and each decision you will face is represented by it forming branches left and right. Each time you make a decision you journey down a particular branch until you reach the next decision. These branches would obviously culminate in billions of final scenarios dependent upon the choices you made along your way through life. Bear in mind that your metaphorical tree is influenced by the decisions taken by other people, too. If another person takes a decision that influences your futures tree it would by necessity be different to if their decision did not influence it. Also bear in mind that we make hundreds of decisions every single day, from which sock to put on first to whether to travel by car or bus, which TV show to watch – everything can, like the butterfly effect, potentially cause subsequent potential future scenarios and options to change. So we are talking about a lot of branches on this tree!
Remember, for this scenario theists argue that god doesn't know which decision we'll make but does know the outcome, he knows all possible outcomes and all possible futures. Yet for this to be the case those other futures must have potential to exist, we must have the ability to choose the branches that lead to them. Correct?
So how can god be omniscient, all knowing, if he doesn't know which branch we'll take at any particular point? Omniscience dictates that he knows everything, which would include those decisions. If he does know then, as demonstrated in the pill example, we cannot logically choose the other branch as it would contravene his foreknowledge – this would effectively prune the tree back to one line we could not deviate from.
He cannot possibly know all future possibilities and also know our every decision beforehand. Knowing our decisions beforehand logically doesn't allow for potential futures that include decisions other than he has foreseen, and without that possibility we cannot have true free will, only the illusion of it.
One way around this I have seen is that god exists outside of space and time. Aside there being no evidence he even exists much less does this, the end result is the same. We live our lives in space and time, our lives and decisions play out in a linear manner. Therefore even if god is able to go back and forth, be every time at once or anything else, he still either does know our every decision before we make it, negating free will, or he doesn't, negating his omniscience.
Hopefully this has made some level of sense, I'm typing at 3am and with a headache.