This is something that is so easy to do when in a discussion with someone. You know, your conversation is heading one direction, then one of you say something that causes it to take a hard left. In normal conversation, this is not a problem; in fact, it makes conversations lively and lengthy. However, in a conversation that involves one or both of the parties defending a position, there are a few things to look out for from your "opponent" and yourself.
They are called logical fallacies. It is advantageous to be able to recognize when your opponent is using one. It is also to your own benefit to ensure you don't use them either. In other words, be able and willing to call "Bull Crap!" on yourself and your opponent.
I'm going to quickly discuss four of the most common pitfalls.
The Genetic Fallacy
The genetic fallacy is committed anytime the truth of a claim is based on its origin. A common example is believing something is true (such as your religion) because your parents said it is true. Though it may be true, just because your parents say that it is, does not make it so.
This fallacy is commonly committed in a debate when an opponent (confronted with solid evidence) says something along the lines of, "You just believe that because that's how you were brought up," or "people believed that hundreds of years ago." From either of these, they try to make the conclusion that what you are defending can't possibly be true. But that does not follow.
If this fallacy were to be committed by your opponent, you can neutralize the attack by applying the exact same reasoning to something that both of you would agree is ridiculous. By way of the examples above, good responses might sound something like these, "I was brought up to believe the earth is a sphere. Should I not believe that either?" and "Hundreds of years ago people believed the sun was the center of the solar system. Am I wrong to believe that also?" Then explain why their attack is useless.
The ad hominem Attack
This is a fallacious attack on the person presenting the argument. Most of the time this takes place in the shape of an attack on the character or the credibility of the person presenting the argument.
Character and credibility have nothing to do with whether or not a conclusion is correct. We would all agree that Hitler is probably one of the persons with the worst character in history; however, if he told us that adultery is wrong, we would be committing an ad hominem attack if we pointed out that he killed over 6 million people, then concluded that we couldn't trust him on moral issues. We know that adultery is wrong, but we could not defend our position based on the character of the person making the argument (Hitler). The reason for this is because someone else whom we may consider to be of a much higher character than Hitler may make the exact same argument, and our counter-argument that we used against Hitler would not work against the new opponent.
The key to preventing yourself from using this kind of attack is to ask yourself if your argument (or counter-argument) could be used no matter who you were arguing against.
If this type of attack were to be made by your opponent, you may want to question its validity by asking them to defend their claim (that your character somehow makes your argument invalid). If they are placed back on the defensive, they are placed into the uncomfortable position of defending a personal attack. They may also accuse you of trying to divert the subject by asking them to defend this position, but you may reply by pointing out that an attack on your character was the diversion first attempted by them- you are just going with the flow.
Check out the Wikipedia article about ad hominem attacks.
The Straw Man Fallacy
The Straw Man fallacy is a favorite among all people. What's tricky about this one is that it can be committed knowingly or unknowingly.
The Straw Man fallacy basically involves misrepresenting the opposing argument in such a way that it is easily destroyed by your counter-arguments. A lot of times, someone setting up a "straw man" will "forget" an important piece of or over-simplify the opposing argument.
The people who commit this fallacy unknowingly typically just haven't done their homework in respect to the position they are arguing against. If someone ever says "You are misrepresenting the facts of my position", you have committed this fallacy unknowingly; you need to admit it, and promise to do more research to adjust your argument to be fighting the "real issue" not a fake issue that is in your head.
The people who commit this fallacy knowingly typically do it without the opposition to correct them. This allows the people attending to "see" why the arguments of the opposition are useless. But, when one of the attendees confront someone of the opposing position, they will be made to be a fool, because of the original presenter's "straw man" representation of the opposing argument.
This fallacy is so easy to fall into. This is why we need to make certain that we understand the opposition's position and the arguments they use to reach their conclusions. Don't spend time fighting against something that is not the actual issue (just a bad representation of it) (the "straw man"). It is best to find the strongest arguments for the opposition and argue against those. If you can successfully argue against your opponent's strongest evidences, you will do considerable damage to their position.
Common Belief Fallacy
I have seen way too many people claim that because "many" people believe something, that makes it true or at least viable. However, this does a great disservice to the definition of "truth". If a "common belief" were to hold any amount of weight for determining "truth", then "truth" would be reduced to being relative to how whims of the people. Absolute "truth" would then never be able to be known, thus destroying any reason for even arguing about the truth of anything, because it might change in the next couple years.
I would also like to know if "many" is a reference to number or percentage. If number, the relative to what other number to define it as "many"? What is the significance of using the word "many" anyway? Why not "some" or "a few" or "tons" or no quantitative (referring to number of) adjective at all? The only conclusion that can be arrived at by a claim of this sort (regardless of the adjective) is that it is in the minds of people, thus needs attention. But nothing more.
Note to All about Fallacies
By committing any of these fallacies, we commit academic dishonesty in our arguments and lose credibility among our peers, the public, and those we wish to debate.
Note to Christians about Fallacies
As a Christian, this is extremely important because the limit of our credibility will be the limit to which people will listen to our message. Also, some people will project our limited credibility onto others who hold our same position (whether or not the others have made the same mistake we did of committing these fallacies). As Christians we should understand the eternal damage we could be causing by even flirting with one of these fallacies.
Now, I'm not trying to say that I never have or never will commit one (read as "all") of these fallacies. I will make mistakes, but when I do, I back-peddle as quickly as I possibly can to maintain my (and my peers') credibility.
Here's a good blog post about intellectual honesty and the Bible:
Integrating Argument and Virtue
For more information about these fallacies and others, Norman Geisler has a great book Come, Let Us Reason.
Here are a couple episodes of Straight Thinking by Kenneth Samples in which he discusses logic and touches a bit on these fallacies.
Need for Dispassionate Analysis
Avoiding the Straw Man
Avoiding the ad-Hominem Attack