I’d be willing to put $500 on the line right now that I can give you a valid and sound deductive argument, where the conclusion renders the truth about whether God exists. I hereby guarantee that one of the following two arguments fits that description:
1. God exists.
2. Therefore, God exists.
1. God does not exist.
2. Therefore, God does not exist.
Each of these argument are such that if their premises are true, it is impossible for their conclusion to be true. This makes the arguments logically valid. Furthermore, it is necessarily the case that one of these arguments has a true conclusion, meaning therefore that one of them is sound.
Why don’t these arguments impress us in the slightest? It’s because validity and soundness are not the only measures of argument quality. A commonly ignored factor is non-circularity.
Most circular arguments are not as blatant as these. Often a long, drawn out argument will have as one of its premises a stylistic variant on the exact conclusion the argument is trying to make. Sometimes this is further obfuscated by some other cleverly disguised logical fallacy, such as an equivocation. This happened in my epistemology class last week – we removed an equivocation in terms from an argument only to find that its seventh premise was just a fancy way of saying exactly the same thing as its conclusion. Circular argumentation is sometimes known as begging the question.
Since circular arguments are neither helpful nor persuasive to anybody, philosophers would do best to avoid them at all costs. Examine your arguments carefully. Look for any premise that depend upon the conclusion already being true for its justification, then try to remove the circularity by justifying your premises on outside grounds. You may come up with brand-new arguments that will be all the stronger for having been through this process.
For more on circularity, check out this page from The Fallacy Files, the best website I’ve come across dealing with various types of logical fallacies.