About Check My Logic

Check My Logic is a reference to the ways reasoning goes wrong: formal and informal fallacies, cognitive biases, and research biases (flaws in how evidence is gathered and reported). 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. Research biases belong to studies: how a sample was drawn, how results were analyzed, which ones got published. “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 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.

If you know what happened but not what it’s called, you can describe it to Portia, the AI assistant made by Arachnida Apps, the same people who make this site. It points you to the page, or to the few closest to what you described, as cards at the top of the page. The name and summary on each card come from this site; the one-line reason under them is Portia’s, and is marked as AI. The entry itself is still the reference.

The Portia button appears to readers in the United States. Using it needs an Arachnida account, and it is off until you turn on AI features there (which also asks you to confirm you are 18 or over); if you’re signed out, the button offers to sign you in and brings you back to the same page. If you’ve turned AI features off, the button doesn’t appear. Check My Logic works exactly the same without it.

To decide whether to show that button, each page asks Portia, from your browser, whether Portia is available to you. If you’re signed in, that request carries your Arachnida sign-in and tells Portia you have Check My Logic open, but not which entry you’re reading.

The panel that opens is Portia’s own page, not part of this site. What you type there goes to Portia, not to Check My Logic, and Portia sends it to Anthropic, the company whose models it runs on, to write its replies. It is kept as one of your Portia chats, under the Arachnida Apps privacy policy. Check My Logic receives only the pages Portia suggests, and keeps them in this browser tab until you dismiss them.