Inconsistency, Irrelevance, Insufficiency, and Inappropriate Presumption

Read these four tutorials, which introduce four major classifications of fallacies. Although there are many possible ways of categorizing fallacies, the four major groups discussed in these tutorials are fairly standard.


Fallacies of inconsistency are cases where something inconsistent, self-contradictory or self-defeating is presented.

§1. Inconsistency

Here are some examples:

    • "One thing that we know for certain is that nothing is ever true or false." – If there is something we know for certain, then there is at least one truth that we know. So it cannot be the case that nothing is true or false.

    • "Morality is relative and it is just a matter of opinion, and so it is always wrong to impose our opinions on other people." But if morality is relative, it is also a relative matter whether we should impose our opinions on other people. If we should not do that, there is at least one thing that is objectively wrong.

    • "All general claims have exceptions." – This claim itself is a general claim, and so if it is true, it must also have an exception itself. This implies that not all general claims have exceptions. So the claim itself is inconsistent.

§2. Self-Defeating Claims

A self-defeating statement is a statement that strictly speaking is not logically inconsistent, but is near enough in that it is obviously false when being asserted. Consider these examples:

    • Young children are fond of saying "I am not here" when they are playing hide-and-seek. The statement itself is not logically inconsistent, since it is logically possible for the child not to be where she is. What is impossible is to utter the sentence as a true sentence (unless it is used for example in a telephone recorded message.)

    • Someone who says, "I cannot speak any English."

    • Here is an actual example. A TV program in Hong Kong was critical of the government. When the Hong Kong Chief Executive Mr. Tung was asked about it, he replied, "I shall not comment on such distasteful programs." Mr. Tung's remark was not logically inconsistent, because what it describes is a possible state of affairs. But it is nonetheless self-defeating because calling the program "distasteful" is to pass a comment!

Source: Joe Lau and Jonathan Chan,
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