We all argue... but how do you know whether an argument is valid?
There are several ways to get what we want. Fighting, stealing... but for most of us, persuasion is a more common approach.
Whether we are negotiating a salary, giving our views on abortion or the death penalty, or arguing over who should wash the dishes, persuasion by argument is central to our lives. Indeed, arguing is an essential part of what it means to be human.
So what is an argument?
An argument can be broken down into a premise (or premises) and a conclusion. For example, the traditional "pro-life" position on abortion consists of two premises followed by a conclusion:
Premise one: "It is wrong to deliberately kill an innocent human being"
Premise two: "A foetus is an innocent human being"
Conclusion: "Therefore, deliberately killing a foetus is wrong"
Is it a valid argument? An argument is valid if the premises lead logically to the conclusion. If the premises are true, then the conclusion must be true.
Since the validity of an argument rests solely on the connection between the premises and the conclusion, an argument can be valid even though the premises and conclusion are false.
Take this as an example:
Premise one: "Iraq has weapons of mass destruction (WMD)"
Premise two: "WMD must be destroyed"
Conclusion: "Therefore, we should destroy Iraq's WMD"
Will logic or passion win the day?
This is a valid argument, since the conclusion follows from the premises. Yet it is not a sound argument - because that would require both that the argument be valid and that the premises be true, and we now know that Iraq had no WMD. Premise one turned out to be false.
And there are other perspectives to consider. Returning to the pro-life argument, an opponent might question the premise that it is wrong to deliberately kill an innocent human being by asking what we mean by "innocent".
If the foetus threatens the woman's life, is it still innocent? Another move is to deny that abortion is always a deliberate killing of the foetus. When a doctor removes the uterus to treat a malignant tumour, the intent is not to kill the foetus - its death is a side-effect of the medical procedure.
So how can we identify whether an argument is built on sound foundations. How do we detect fallacies?
Fallacies are defects in our arguments. They are likely to crop up when we feel strongly about an issue, when we construct an argument on the spur of the moment, or when we are factually ignorant of the subject.
A common fallacy is to attack the person making the argument, instead of the argument itself: "We should reject Mr Smith's views on the death penalty, however appealing they may be, because Mr Smith has been addicted to cocaine and alcohol for many years."
Rather than examine the soundness of the argument, the critic diverts attention away from the argument to Mr Smith's socially unacceptable lifestyle. This is an example of the ad hominem fallacy
("against the person").
Another common fallacy is the appeal to authority, which consists of arguing a point by invoking the opinion of an expert. However, experts may be wrong, they may be expressing an opinion outside their area of expertise or they may have been incapacitated or joking when making the point. It is the expert's reasons that are valuable, not the fact that they were announced by an expert.
It is also tempting to make broad generalisations based on a small sample. We notice one threatening ruffian with a pony-tail and immediately believe all pony-tailed youths are thugs.
This is the fallacy of the lonely fact
. When studying for my PhD, I interviewed people who believed doctors should not disclose a grim prognosis to patients. They based their views on anecdotes about patients who committed suicide or died very soon after such disclosures.
They derived a broad conclusion from a tiny sample. At the other extreme, some people believed doctors should tell patients "the whole truth", however ghastly.
The black-and-white fallacy
refers to the belief that there are only two possibilities - conceal the truth from the patient or disclose everything - when other alternatives exist, such as revealing information gradually and assessing whether the patient requires more.
In ethics, people sometimes invoke the slippery slope argument. The idea is that if you allow one thing to happen, it will trigger a chain reaction that will ultimately lead to a terrible state of affairs.
Opponents of euthanasia may argue that legalising morally acceptable cases of euthanasia will lead to legalising unacceptable ones. Some forms of slippery slope argument can be compelling, since the slide from the top of the slope to the bottom is obvious. In other cases, the slope may be less slippery. Some slippery slopes are akin to a series of manageable steps rather than a soap-covered slide into the jaws of evil.
Those familiar with internet culture may have heard of Godwin's law. Coined by the American lawyer Mike Godwin in 1990, it states that the greater the length of an internet discussion, the higher the chances of a comparison involving Hitler or the Nazis.
The law reflects the tendency of some online forum users to use slippery slope or ad hominem arguments to win often impassioned discussions.
Many fallacious arguments are persuasive, and an accomplished speaker can deliberately mislead others through the subtle use of fallacies.