Formal logic is awesome. Since Frege in the late 1800s, logic has really got its big boy boots on and has gone off into weird and magical worlds which more name-droppy bloggers would name.
Formal logic (by and large) is concerned with truth-preservation. If A is true and B is true, the conclusion, C, must be true. There is no consistent interpretation of A, B, and C such that, if A and B are true, C is false. And so for the past hundred and umpteen years, logicians have tried to work out ways of better preserving truth (which is awesome: they make Truth Jam and Nintendo will soon release Truth Jam Sessions on the DS).
Informal logic (by and large) is concerned with justification. Nevermind if A is true; are you justified in believing A? It might be true that there is a many-headed monster eating children in the park (hooray!), but I might not be justified in believing that there is a many-headed monster eating children in the park until certain conditions have been met. These ‘certain conditions’ are going to be based on socially constructed rules, and you’ve probably all heard of them: ‘Ad hominem’, ‘Ad populum’, ‘Tu quoque’, and other such pretentious Latin terms tend to refer to these rules.
The problem, as I hinted earlier, is that these are now about justification and not about truth. Truth is easy in comparison. Something either is true or something is false. Justification, on the other hand, is significantly murkier.
It is either true or false that you believe that it is too warm. While your interpretation of ‘too warm’ is subjective, it is objectively the case that you either hold the belief or that you do not. If you tell me that you think it’s too warm, I’m justified in believing that you hold the belief that it’s too warm. You might be lying, of course, so it might be a false belief. This shouldn’t worry us: there are going to be times when you’re justified in believing something which turns out to be false (and we do it a lot in real life).
Was I really justified in believing that you are too warm? As the Internet will tell you, the truth of a proposition has no relation to the author of the proposition [for those of you who are concerned about the above example (probably Erik and Lane) in relation to where this ramble has gone: remove the pronouns and replace them with names – the problem remains]. When it comes to statements about your mental world, you have an authority which no other person can trump. Therefore, while the truth of a proposition has no relation to the author of the proposition, you are justified in believing that somebody is warm if they tell you that they are warm, even if this is listed on the internet as some sort of fallacy.
Which brings me rather nicely to my point. These ‘fallacies’ aren’t hard and fast rules. Indeed, they aren’t really rules at all. They’re shorthand for a much more complex process: reasoning about reasoning. The ‘Ad hominem’ is not a fallacy, but we can understand that, in general, the advocate of a proposition might be considered insufficient by a great many people for justifying a belief in some proposition.
This shouldn’t shock us. De Morgan’s Laws, for example, are usually memorised by every logic student, but — when it comes down to it — they’re just shorthand for understanding the mechanics of the operators. If you understand how the operators work, you don’t really need to remember de Morgan’s Laws.
Similarly, if you understand how reasoning works and you can formulate arguments about what kind of reasons are good reasons, you don’t need to memorise a painful list of so-called ‘fallacies’ (I say ‘so-called’ because a rather obscene number of ‘fallacies’ originate on Wikipedia. Trufax: several websites mirror Wikipedia and a larger number cite Wikipedia — grumble. Thus, the pages on Wikipedia regarding most folk-philosophy topics — such as informal logic, &c. — become ‘encyclopaedic’ by rapidly replicating through the blogosphere and mirror-pages, racking up the hit count for the Googlewhackers. This is considered ‘cool’ by Wikinerds and they are wrong).
In summary, truth and justification are not the same thing: you might not be justified in believing a true proposition or you might be justified in believing a false proposition. While informal logic is interested in justified beliefs (because it is interested in how people reason), formal logic is interested in truth preservation (because it’s awesome). As informal logic is interested in how people reason, so-called ‘fallacies’ are going to be culturally relative (what’s considered justified to one person might not be considered justified to another — consider the sentence ‘I ate the beef because beef is delicious’ being analysed by a vegan and a non-vegan). Instead of worrying about these informal fallacies, we should worry about the reasoning process itself instead of reducing justification to a rather dreary, rule-based game.