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What is a logical fallacy?

All arguments have the same basic structure: A therefore B. 

They begin with one or more premises (A), which is a fact or assumption upon which the argument is based. They then apply a logical principle (therefore) to arrive at a conclusion (B). An example of a logical principle is that of equivalence. For example, if you begin with the premises that A=B and B=C, you can apply the logical principle of equivalence to conclude that A=C. A logical fallacy is a false or incorrect logical principle. An argument that is based upon a logical fallacy is therefore not valid. It is important to note that if the logic of an argument is valid then the conclusion must also be valid, which means that if the premises are all true then the conclusion must also be true. Valid logic applied to one or more false premises, however, leads to an invalid argument. Also, if an argument is not valid the conclusion may, by chance, still be true.

Top 20 Logical Fallacies (in alphabetical order)

- Introduction to Argument
Structure of a Logical Argument Whether we are consciously aware of it or not, our arguments all follow a certain basic structure. They begin with one or more premises, which are facts that the argument takes for granted as the starting point. Then a principle of logic is applied in order to come to a conclusion. This structure is often illustrated symbolically with the following example: 

Premise 1: If A = B, Premise 2: and B = C Logical connection: Then (apply principle of equivalence) Conclusion: A = C 

In order for an argument to be considered valid the logical form of the argument must work – must be valid. A valid argument is one in which, if the premises are true, then the conclusion must be true also. However, if one or more premise is false then a valid logical argument may still lead to a false conclusion. A sound argument is one in which the logic is valid and the premises are true, in which case the conclusion must be true. 

Also it is important to note that an argument may use wrong information, or faulty logic to reach a conclusion that happens to be true. An invalid or unsound argument does not necessarily prove the conclusion false. Demonstrating that an argument is not valid or not sound, however, removes it as support for the truth of the conclusion – it means that the conclusion is not necessarily true. 

Breaking down an argument into its components is a very useful exercise, for it enables us to examine both our own arguments and those of others and critically analyze them for validity. This is an excellent way of sharpening one’s thinking, avoiding biases, and making effective arguments. 

Examine your Premises 

As stated above, in order for an argument to be sound all of its premises must be true. Often, different people come to different conclusions because they are starting with different premises. So examining all the premises of each argument is a good place to start. 

There are three types of potential problems with premises. The first, and most obvious, is that a premise can be wrong. 

Another type of premise error occurs when one or more premises is an unwarranted assumption. The premise may or may not be true, but it has not been established sufficiently to serve as a premise for an argument. Identifying all the assumptions upon which an argument is dependent is often the most critical step in analyzing an argument. Frequently, different conclusions are arrived at because of differing assumptions. 

Often people will choose the assumptions that best fit the conclusion they prefer. In fact, psychological experiments show that most people start with conclusions they desire, then reverse engineer arguments to support them – a process called rationalization. 

One way to resolve the problem of using assumptions as premises is to carefully identify and disclose those assumptions up front. Such arguments are often called “hypothetical,” or prefaced with the statement “Let’s assume for the sake of argument.” Also, if two people examine their arguments and realize they are using different assumptions as premises, then at least they can “agree to disagree.” They will realize that their disagreement cannot be resolved until more information is available to clarify which assumptions are more likely to be correct. 

The third type of premise difficulty is the most insidious: the hidden premise. I have seen this listed as a logical fallacy – the unstated major premise, but it is more accurate to consider it here. Obviously, if a disagreement is based upon a hidden premise, then the disagreement will be irresolvable. So when coming to an impasse in resolving differences, it is a good idea to go back and see if there are any implied premises that have not been addressed.