Logical fallacies occur in arguments wherein the conclusion does not follow from the premises. This is independent of whether the premises are true or false.

It is quite possible to draw an errant conclusion from all true premises. This happens all the time. Here is an example:

1. If it is raining, then the sidewalks are wet.

2. The sidewalks are wet.

3. Therefore, it is raining.

After reading the above argument, consider the situation represented in this picture:

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What all fallacious arguments have in common is that they overlook some way that their conclusion could still be false even if their premises were all true.

When there are not any fallacies in an argument, it means that the conclusion really must be true if the premises are. In such a case we can say that the argument is valid; otherwise we can say it is invalid.

Note that an argument's being valid does not mean that its conclusion automatically is true; it means merely that the conclusion must be true supposing that all the premises are true. If in addition to being valid, all the premises are indeed true, then we can say the argument is also sound.

Often the premises of an argument are basic facts of empirical observation, such as "It is raining." Other times, they are statistical facts, such as "Auto accidents in the USA are up more than 5% this year." And other times they are ideals or principles that go beyond any simple empirical test, such as "Murder is wrong" or "Space is infinite".

Fallacy checking by itself does not test the accuracy of the premises. It only asks, if we assume the premises to be true ("for sake of argument"), then does the conclusion follow necessarily? Answering this is a separate task from the equally important task of checking the accuracy of facts asserted in the premises. There are many sources of fact checking today, but Fallacycheck.com was created to focus on checking for fallacies.

When we find a fallacy, or a possible fallacy, it only means that the one particular argument in question is not a valid argument. It does not mean that every argument the author made (authors often make more than one argument in an article) was fallacious. And it does not mean there do not exist other arguments, perhaps by other authors, that argue for the same conclusion, but without fallacy. It just means that this one particular argument is not a valid argument for that position.

The absence of fallacies, given the explanation above, does not mean the arguer is "right". In other words, the arguer's conclusion could still be mistaken if one or more of the starting premises turns out to be false. But the presence of a fallacy indicates that someone wanting to support the conclusion needs to seek for a better argument.

Writers do not always write clearly (and sometimes this is on purpose). This is why, in some cases, our description will say there is a "possible" fallacy, or "if the author's point is...." then there is a fallacy. Some authors will imply a fallacious argument without making the conclusion wholly explicit. This may even be an intentional tactic, so that if the fallacy is called out, the author can then retreat, and reply "But that is not what I meant". In most cases, however, writers are clear enough in their statements that the presence of a fallacy can be determined without ambiguity. The final judgment, however, is up to you, the reader. This is why we provide explanations of each instance of a fallacy, so that you can consider our explanation, then re-read the original text yourself. Our purpose is to aid you in seeing the fallacy, and how it is constructed, as you read the source itself. If you don't, then let us know! We always value feedback on the accuracy and helpfulness of our service.

To learn more about how we check texts for fallacies, read about the principles of charity and fidelity.