I thought this was an interesting video on optical illusions from TED Talks.
“This isn't an argument to undesirable consequences; though the consequences of denying such should be undesirable for the rational person. When an argument is about a certain proposition, it is reasonable to assess the truth-value of any logical consequences of it. Logical consequences should not be confused with causal consequences, and truth or falsity should not be confused with goodness or badness. Since God is the necessary precondition for the intelligibility of reality, then it follows that one couldn't prove anything, much less His existence without first assuming His existence since His existence is the very thing needed for the concept of proof in the first place. Thus, the proposition itself (i.e., the existence of God) is in question, not the logical relation of the proposition to the intelligibility of reality or the issue of whether the consequences are desirable or not. “
“Everybody reasons in a circle and doing so isn't always fallacious. There are two things we need to discuss about circular reasoning: It is (1) absolutely unavoidable and (2) not necessarily fallacious. Circular reasoning is unavoidable to some degree when proving one's ultimate standard. An ultimate standard cannot be proved from anything else, otherwise it wouldn't be ultimate. Therefore, if it is to be proved, it must use itself as its own standard of judgment by which any decision is made.”
“Second, all circles aren't necessarily fallacious. Begging the question is often considered a fallacy because it is usually arbitrary. But it can be non-arbitrary if it goes beyond a simple circle (i.e., the Bible is true because it says so) and uses additional information to support its conclusion. If the ultimate authority is first assumed and you find out later you have good reasons for it because without it you cannot make sense out of anything, then its perfectly legitimate to reason in a circle.”
“In fact, any true ultimate authority must use itself as part of its own proof. Again, some degree of circular reasoning is involved, but it cannot be a simple "vicious" circle. It must be non-arbitrary. Consider logic:
1 - If there were no laws of logic, we couldn't make an argument.
2 - We can make an argument.
3 - Therefore, there must be laws of logic.
This argument is perfectly sound yet it is subtly circular. It's what is known as a modus tollens syllogism (i.e., denying the consequent) and in this "proof", we have assumed that there are laws of logic. Modus tollens is a law of inference in logic, and we have used it as part of the proof that there are laws of logic. In this case we had no other choice; in order to get anywhere in any argument we must presuppose that there are laws of logic.”
“However, this example argument doesn't merely assume what its trying to prove; it imports additional information to support its conclusion. What makes this circular argument a powerful one is that to deny it would be to assume it, thus any potential rebuttal would be self-defeating. A great way to show that a particular presupposition must be true is to show that one would have to assume that the presupposition is true even to argue against it in the first place. “
“The Christian's ultimate standard is like this; any attempt to refute the Bible must assume things about the world that could only be true if the Bible were true in order to get started. The Bible not only provides the criteria for itself, but it does so for all other facts, hence, the reasoning isn't viciously circular. It gives us a foundation (the Biblical God) for rational reasoning (including laws of logic), science, morality, reliability of our senses and memory, and so on.”
“As with the argument for laws of logic, any attempted rebuttal would be self-refuting, because it would have to use things (laws of logic, the charge to be consistent, etc.) that presuppose a universe that can only exist if Christian theism is true. Thus, we are not merely arguing "The Bible must be the word of God because it says so". Rather, we are saying, "The Bible must be the word of God not only because it says it is, but if you reject this claim you are reduced to absurdity."”