A logical fallacy occurs when an argument contains flawed reasoning. These arguments cannot be relied on to make truth claims. There are two general kinds of logical fallacies: formal and informal.

First off, let’s define some terms.

  • Argument: a group of statements made up of one or more premises and one conclusion.
  • Premise: a statement that provides reason or support for the conclusion
  • Truth: a property of statements, i.e. that they are the case
  • Validity: a property of arguments, i.e. that they are logically structured
  • Soundness: a property of statements and arguments, i.e. that they are valid and true
  • Conclusion: the final statement in an argument that indicates the idea the arguer is trying to prove

Formal logical fallacies

These are arguments with true premises, but a flaw in its logical structure. Here’s an example:

  • Premise 1: In summer, the weather is hot.
  • Premise 2: The weather is hot.
  • Conclusion: Therefore, it is summer.

Even though statement 1 and 2 are true, the argument goes in circles. By using an effect to determine a cause, the argument becomes invalid. Therefore, statement 3 (the conclusion) can’t be trusted.

Informal logical fallacies 

These are arguments with false premises. They are based on claims that are not even true. Even if the logical structure is valid, it becomes unsound. For example:

  • Premise 1: All men have hairy beards.
  • Premise 2: Tim is a man.
  • Conclusion: Therefore, Tim has a hairy beard.

Statement 1 is false – there are plenty of men without hairy beards. Statement 2 is true. Though the logical structure is valid (it doesn’t go in circles), the argument is still unsound. The conclusion is false.

A famous example of an argument that is both valid, true, and sound is as follows.

  • Premise 1: All men are mortal.
  • Premise 2: Socrates is a man.
  • Conclusion: Socrates is mortal.

It’s important to look out for logical fallacies in the arguments people make. Bad arguments can lead to true conclusions, but there is no reason for us to trust the argument that got us to the conclusion. We might have missed something or it might not always be the case.