About Check My Logic

Check My Logic is a reference to the ways reasoning goes wrong: formal fallacies, informal fallacies and cognitive biases. Entries are written to be understood in a minute and trusted after ten: a short definition and worked examples up front, with look-alikes, the form, variants, causes and evidence one click away.

It’s also a website whose name is something people say right before they get defensive. We’re aware.

Strictly, only formal fallacies are failures of logic. Informal fallacies are failures of relevance, assumption or meaning, and cognitive biases aren’t properties of arguments at all. They’re properties of the people making them. “Check My Reasoning” would be more accurate.

But “check my logic” is the phrase people actually use, and a reference site that names its own category error on the about page is, at least, being consistent.

The most common misuse of a list like this is to find the name for what someone did and consider the argument won. It isn’t. An argument with a flaw can still reach a true conclusion; it just doesn’t show that conclusion. Concluding that a claim is false because an argument for it was bad is its own error.

That’s why every entry has a section called When it isn’t an error. Plenty of moves that look like fallacies are reasonable in context: questioning a witness’s honesty, treating a result as evidence for its likely cause, trusting a general impression when it’s all you have. The useful question is never “does this have a name?” but “does this flaw actually undermine the conclusion here?”

Fallacies are about the structure and content of arguments, so they are settled by analysis. Biases are claims about human psychology, so they are settled by experiment, and several famous ones have not survived other researchers trying to repeat them. Every bias here is labeled:

• Replicates robustly: found again and again, by independent researchers, using different methods.
• Contested: genuine disagreement about whether it exists, how large it is, or what causes it.
• Largely failed to replicate: careful attempts to repeat the key findings mostly haven’t found it. It stays listed, because people still cite it.

A site called Check My Logic repeating a debunked finding as settled fact would be an especially embarrassing way to be wrong.

Entries belong to one or more categories: some errors are both a fallacy and a bias. Different names for the same error are listed as aliases on a single entry rather than as separate entries. Underlying ideas aren’t errors. Some are valid forms, such as affirming the antecedent (modus ponens), the correct patterns that formal fallacies imitate, often under confusingly similar names. Others are principles, such as what you see is all there is, that explain why errors happen.

Entries are added one at a time, and only once they’re understood well enough to explain. That’s why the list is short.