A fallacy is just a mistake in reasoning. Humans are not nearly as rational as we’d like to suppose. In fact we are so prone to certain sorts of mistakes in reasoning that philosophers and logicians refer those mistakes by name. For now I will discuss just one by name but in a little detail. Watch for explanations of other fallacies over the course of the class. For pretty thorough catalogue of logical fallacies, I’ll refer to you The Fallacy Files (http://www.fallacyfiles.org/taxonomy.html).
“Ad hominem” is Latin for “against the man.” It is the name for the fallacy of attacking the proponent of a position rather than critically evaluating the reasons offered for the proponent’s position. The reason ad hominem is a fallacy is just that the attack on an individual is simply not relevant to the quality of the reasoning offered by that person. Attacking the person who offers an argument has nothing to do whether or not the premises of the argument are true or support the conclusion. Ad hominem is a particularly rampant and destructive fallacy in our society. What makes it so destructive is that it turns the cooperative social project of inquiry through conversation into polarized verbal combat. This fallacy makes rational communication impossible while it diverts attention from interesting issues that often could be fruitfully investigated.
Here is a classic example of ad hominem: A car salesman argues for the quality of an automobile and the potential buyer discounts the argument with the thought that the person is just trying to earn a commission. There may be good reason to think the salesman is just trying to earn a commission. But even if there is, this is irrelevant to the evaluation of the reasons the salesman is offering. The reasons should be evaluated on their own merits. Notice, it is easy to describe a situation where it is both true that the salesman is just trying to earn a commission and true that he is making good arguments. Consider a salesman who is not too fond of people and cares little for them except that they earn a commission for him. Otherwise he is scrupulously honest and a person of moral integrity. In order to reconcile himself with the duties of a sales job, he carefully researches his product and only accepts a sales position with the business that sells the very best. He then sincerely delivers good arguments for the quality of his product, makes lots of money, and dresses well. This salesman must have been a philosophy major. The customer who rejects his argument on the ad hominim grounds that he is just trying to earn a commission misses an opportunity to buy the best. The moral of the story is just that the salesperson’s motive is logically independent of the quality of his argument.