I was listening to a snippet of a debate this morning between the atheist Dan Barker and a Christian whose name I do not recall (sorry). Throughout much of the cross examination portion of the debate, Barker kept appealing to the audience with statements that essentially stated "you all know this is true" or "I know you would agree with me when I say ..." (While these are not exact quotes, they present the essence of what Barker was saying.) The tactic is a common one when trying to convince someone else of your opinion. The problem is, whether or not someone already agrees with you is entirely irrelevant to the truth or falsity of the point you are arguing for. Truth is not, after all, determined by majority rule.
Let me offer a more specific example of irrelevance - one that I see presented over and over again in debates between atheists and Christians.
Quite often a statement such as "well you believe in a book that was written thousands of years ago by people lacking the scientific insights we have today" is presented as a counter to a Christian's appeal to the Bible as a useful source of knowledge. The statement is typically made in passing, so as to not draw too much scrutiny, but I fear it often has it's intended impact nonetheless - namely to discredit the authority that the Christian is drawing on in their argument. The problem with this type of assertion is twofold.
First, it is irrelevant to the question normally under debate, namely whether or not the Christian God exists. Recall that the Christian's position is that God himself inspired these writers to write the words they penned, meaning that it wouldn't matter how much scientific knowledge the writers possessed. All they would need to be able to do is to write what God inspired them to write. The atheist that happens to follow this line of reasoning demonstrates either a lack of understanding of the claims that Christians make, or a complete disregard for the lack of relevance of the argument.
Second, it can be used against the atheist's own position. There are, as expected, examples of atheist writers in the far past - some as far back as 600 BCE. If not possessing the scientific knowledge we have today was a shortcoming of those penning the scripture of the OT and NT, then why would it also not be a shortcoming of those who held to a non-theistic position? To claim that Theistic writings during this time period are suspect due to lack of present-day scientific knowledge, but non-Theistic writings are not, is to hold a double-standard and be guilty of the fallacy of Special Pleading.
The fallacies of relevance are numerous - too numerous to cover in a short post on this blog. However, they are something each of us should be aware of. Not only for the sake of correcting others when they use them, but also to be certain that we do not fall into the trap of using them ourselves. We are not only interested in the end, but the means we employ as well.