Introduction

For regulars at Ars Technica, the forums are as much a part of the site's identity as the articles. And where there are forums, there are flame wars. The BattleFront is infamous for its contentious threads, but arguments arise just as easily in The Observatory. What are we to do when when people disagree with one another? Is it possible for one argument to be right while another is wrong? Or is everything just a matter of opinion?

In some cases, there is a way to tell good arguments from bad using what is called informal logic. This name distinguishes it from formal logic, which is used in mathematics; natural language is less precise than mathematics, and does not always follow the same rules. Perhaps more surprisingly, the name also reflects the fact that there is a lot of disagreement over what it means. Informal logic is actually a fairly young discipline, developed in the 1960s, and intended to apply new techniques from formal logic to argument and critical thinking. Philosophers are still wrestling with this application, and there are several competing schools of thought.

For all the differences, there are some core elements upon which everyone agrees. This article is a tutorial introducing these basic ideas of informal logic. In particular, we focus on deductive reasoning, which is one of the cornerstones of analytical thought. Hopefully you will come away from this article with the tools to distinguish a productive debate from an acrimonious flame war.

The fallacy fallacy

A common approach to studying informal logic is to start with logical fallacies. Fallacies are common mistakes in reasoning that are easy to identify. These are the things with funny names that occasionally appear in the forums: ad hominem, post hoc, begging the question, and so on. Conveniently, any forum poster can always provide a link that explains them.

However, fallacies are not necessarily the best place to start learning informal logic. While they can help us identify bad arguments, they are less help in identifying good arguments. Fallacies are often subtle errors, and it is hard to recognize one without knowing what makes a good argument in the first place.

An example of this problem is the ad hominem fallacy. In this fallacy, an argument is refuted by an irrelevant fact about the person making it. For example, Ben Kuchera has recently written several articles on why he thinks the Sony Move is more promising than the Microsoft Kinect. Suppose a forum poster were to complain that the Red Ring of Death has made Ben bitter about Microsoft's hardware. In addition to being false, this claim would likely be an ad hominem attack.

Note that we have not completely let Ben off the hook here; this is probably a fallacy, but not certainly. A claim about Ben's hardware preferences only counts as an ad hominem attack if it is irrelevant. Unfortunately, there are ways in which it could be relevant. Ben's articles are written on the premise that he has played with the hardware while most of the readers have not. This makes him—in a technical sense—an authority; he uses this authority to present arguments like the following:

I've been lucky enough to spend a good amount of time with [Kinect], ... it was clear that this wasn't a technology that was designed with gamers like me in mind.

If a forum poster wants to argue that Ben is not a credible authority, then Ben's platform biases are relevant. There are other reasons why we want to discourage personal attacks; they are difficult to use correctly and can lead to particularly nasty fights when used incorrectly. Once we start arguing about Ben's credibility, we often ignore all of the other claims that are still valid. However, from a purely logical perspective, it is possible for a personal attack to be a valid argument.

Since we have to know what a relevant argument is before we can identify an ad hominem fallacy, this brings us right back to where we started. This is why many other schools of informal logic prefer to start with a more postive approach and concentrate on what constitutes a good argument.

Breaking up an argument

The first step in analyzing an argument is separating premises from conclusions. A conclusion is the purpose of the argument, while a premise is evidence for accepting this conclusion. For example, suppose we made this claim:

John Timmer hates platypuses, because he never writes articles about them.

The conclusion in this case is John's hatred of that misunderstood mammal. The premise, or evidence, for this claim is that he never writes about it.

Premises are an important part of an argument, but we rarely use logic to evaluate them. We evaluate them with other means, like observation. For example, a simple Google search establishes that John has indeed written about platypuses on Nobel Intent. Some premises may be the outcome of a previous argument that we do not want to rehash. Other premises may reflect our trust in the expertise or authority of the person making it; Ben Kuchera's expertise in gaming is a reason to accept his observations about the Kinect.

Conclusions, on the other hand, are evaluated with logic. Logic helps us recognize that an argument is valid, or that the premises are strong enough to support the conclusion. Valid and true are not the same thing. Suppose, as part of our forum post for platypus advocacy, we claim:

Ars has no articles about platypuses because a Google search on "platypus" returns no pages from Ars.

We typically accept (barring problems with Google, or a change in Ars' robots.txt policy) that a Google search is sufficient to find a platypus article on Ars. That makes this a valid argument, even though we have already seen that the conclusion is not true.

The distinction between validity and truth is important. Technically, logic cannot establish truth; logic can only establish validity. Validity is still useful. If an argument is valid and the premises are true, then the conclusion must also be true. Once we know that an argument is valid, the premises are the only possible source of disagreement.

Sometimes, the line between premise and conclusion becomes less clear, such as when an argument follows a chain of reasoning. Consider the following argument:

There are so few games for the Mac that true gamers will never buy them. And without gamers, Macs will never be anything but a niche computer.

The initial premise is the number of games for the Mac, and from that we conclude the Mac's appeal to gamers. This conclusion is then the premise for yet another conclusion: the Mac's niche status. We call these conclusion/premise combos intermediate conclusions. If an argument is valid, we do not have to worry about them; it is only the initial premises that can be in doubt.

A good debate should first establish that all the arguments are valid, and then spend the rest of the time quibbling about the premises. As part of this debate, we may discover that the premises themselves need evidence, making them intermediate conclusions instead of premises. Eventually we should reach a point where either the premises are accepted by everyone, or we have to "agree to disagree."