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Formal Fallacy

This is when you have an invalid form of an argument, where the premises can be true — but the conclusion might not. And even if the conclusion does turn out to be true, it won’t have been reached reliably. Remember! An argument is valid if the conclusion follows directly from the premises and could not be false if the premises were assumed true.

Formal logic is structured, and so when we have formal fallacies, it’s about intra-argument errors and inconsistencies in the structure. In fact, you can even represent the sentences with letters and key logical terms (such as “if-then”, “iif”, “and”) to map out where the argument goes awry.

For example, the fallacy of affirming the consequent, or converse error:

If A — then B
B is true.
Therefore A is true.

(“If my computer breaks, I can’t write my homework. I can’t write my homework; therefore, my computer must be broken”).

Informal Fallacy

This is any other type of flawed argument other than a formal fallacy — they’re more mistakes we can make, even though they can be valid in their logical structure.

For example, the slippery slope fallacy. It’s where the argument involves claiming there’s going to be a ‘domino effect’ if a certain step or occurrence happens. 

The internet meme “Well, that esculated quickly!” is one simple way to remember it!

Here’s a great cartoon that demonstrates the absurdity of this particular informal fallacy, from the Book of Bad Arguments. It’s a great book for taking an animated look through arguments — and a useful resource for learning philosophy in a creative way. An interview with the creator, Ali Almossawi, can be found here.