Logical fallacies are common ways in which arguments can fail. In philosophy, a logical fallacy tends to be a formal fallacy - a flaw in the logical structure of the argument - but in general use, logical fallacies tend to include fallacies that aren't related to the logical structure of the argument. The difference can best be understood by use of an example:
- Either evolution is true or creationism is true.
- Evolution is not true.
- Therefore, creationism is true.
This argument has the form of a reasonable modus tollens argument:
- P ∨ Q
- ∴ Q
If the premises of the argument are true, then the conclusion does follow. But the premises are not true: the first premise in the argument is false as it presents a false dilemma - it is not the case that either evolution is true or creationism is true. For certain versions of creationism, it is the case that they are incompatible with evolution. But if evolution were not true, it would not mean that creationism were true - it might be true, but there are many other options which could also be true. In this case, the argument does not exhibit any formal fallacies, but it does exhibit at least one informal fallacy. Different terms are used to refer to whether the argument works or not: when someone says an argument is valid, it means that if the premises are true, the conclusion is true. That an argument is valid is no guarantee that the argument is sound. For an argument to be sound it must be valid and all of the premises must be true. An informal fallacy may be understood as a common way in which an valid argument can be unsound. As such, you cannot prove them to be invalid - they aren't. What you can do when faced with someone who puts forward an informal fallacy is construct an argument that uses the informal fallacy but changes the premises and conclusions to show by analogy how the fallacy operates (trying hard, of course, to not construct a false analogy in the process).