Four tools, two jobs: find the weak reasoning in a piece of text, or practise spotting it yourself.
Paste a piece of text — an argument, an article, an email, a social post — and the Fallacy Finder identifies the logical fallacies in it, names each one, quotes the passage that triggered it, and explains why the reasoning doesn’t hold. Every named fallacy links to its full reference page.
You can also upload a PDF instead of pasting text.
Treat the output as a second opinion, not a verdict. It's most reliable when a fallacy is structural — a conclusion that simply doesn’t follow from its premises. It is least reliable on anything that depends on context it can’t see: sarcasm, in-jokes, technical shorthand, or an argument that is deliberately quoting a bad argument in order to criticise it.
If a result looks wrong, it may well be. Read the quoted passage and the explanation, and judge whether the objection actually lands.
The Bias Checker works the same way but looks for cognitive biases rather than fallacies — the patterns of thinking that skew reasoning before an argument is even formed, like confirmation bias, anchoring or survivorship bias. It covers more than thirty.
Fallacies are faults in the argument; biases are faults in the thinking. The same paragraph can contain both, which is why the two tools are separate. Running a piece of text through both is a reasonable habit for anything important.
The Fallacy Quiz gives you example arguments and asks you to name what’s wrong with them. It’s the one tool that needs no account — though signing in records your scores and builds a day streak.
Difficulty levels run from straightforward textbook cases to arguments where the flaw is genuinely arguable. A mixed quiz draws from all levels. Your past attempts, with scores and difficulty, are in quiz history.
The Writing Lab is a writing surface with the analysis built in — for drafting an argument and pressure-testing it as you go, rather than pasting a finished piece somewhere else.
Your documents are listed down the left. The Lab autosaves a few seconds after you stop typing, and the status beside the title tells you where a save has got to. Use the ⋮ menu on any document in the list to rename, export or delete it.
Analysis reports the fallacies and biases in what you’ve actually written. On Pro you can pick how it runs:
The reasoning score is a single figure summarising how well the argument in a document holds together, and it’s stored per document so you can watch it move as you revise.
It’s worth being clear about what it is: a rough signal, not a grade. It reflects how much questionable reasoning was found relative to the length and ambition of the piece. A deliberately provocative essay may score lower than a bland one that never claims anything. Use it to compare drafts of the same piece, rather than as a mark out of ten.
Pro accounts can export a document two ways, both from the ⋮ menu:
.txt file of the text, without formatting.On any plan you can always select the text in the editor and copy it out — your writing is never locked in.
Each tool has its own limits on the free plan, and they aren’t measured in the same units. Plans & limits has the full table.