A phrase that get’s thrown around SFN a lot is “logical fallacy”. You’ve seen it a lot, but do you know what it means? Can you tell a fallacy from valid logic?

Fallacies come in two forms: logical and informal. For the most part, when people talk about logical fallacies, they really mean informal fallacies. Logical fallacies are distinct in logical structure, but informal fallacies are almost all the same in terms of structure. here is a popular list of informal fallacies.

There’s an issue, though. Often, it is the case that in an effort to eradicate all fallacious thinking, people misidentify perfectly fine arguments as fallacies. That is somewhat understandable, though, since some fallacies look a lot like legitimate ways to argue.

For example, Modus Tollens is a legitimate argument form, but it looks like a logical fallacy called “Affirming the Consequent”.

Modus Tollens:
If a is true, then b is true.
b is not true.
Therefore, a is not true.

That is perfectly fine and actually very useful. It does, however look a lot like Affirming the Consequent.

Affirming the Consequent:
If a is true, then b is true.
b is true.
Therefore, a is true.

This is fallacious, because b could be true for any number of reasons that have nothing to do with a. Now, if we change up the last line of that fallacy just a little bit, it turns into a legitimate way to argue called “Abductive Reasoning”.

Abduction:
If a is true, then b is true.
b is true.
Therefore, we have reason to think a is true.

Abduction, like induction, is probabilistic and so this move is ok.

Most of the informal fallacies on the list to which I linked above are just examples of the informal fallacy known as “Non Sequiter”.

Non Sequiter:
a is true.
Therefore b is true.

It’s fallacious, because it just doesn’t follow. There’s no reason in the logical structure to conclude b from a being true. A very common (and often misidentified) example is Ad Hominem. Almost everyone on the internet thinks they know what an Ad Hom is, but many of them are wrong.

“You’re wrong, so you’re stupid” is not an Ad Hom. Nor is “You’re wrong, stupid-face!”. However, “You’re stupid, so you’re wrong” is an Ad Hom. Personal attacks are only fallacious if they’re used to conclude that someone is wrong. Other than that, they’re just bad manners. Again, we can even tweak this slightly to make it non-fallacious. If we say, “You’re stupid, so you’re probably wrong.” is indeed fine as it’s not deductive.

There may or may not be more entries regarding critical thinking and logic to follow; I’ve yet to decide.