Wednesday, February 24, 2010

Intelligent design argument - Fallacy of the undistributed middle

The argument for Intelligent Design is logically flawed.

First, let's start with defining syllogism.

Syllogism is a form of deductive reasoning consisting of a major premise, a minor premise, and a conclusion. A successful syllogism is like this.
  • Major premise: All humans are mortal
  • Minor premise: I am a human
  • Conclusion : I am mortal

The fallacy of the undistributed middle is a logical fallacy that is committed when the minor premise in a categorical syllogism is not distributed. It is thus a syllogistic fallacy. More specifically it is also a form of non sequitur (Latin for "it does not follow").

The fallacy of the undistributed middle takes the following form:
  • All Zs are Bs.
  • Y is a B.
  • Therefore, Y is a Z.
It may or may not be the case that "all Zs are Bs," but in either case it is irrelevant to the conclusion. What is relevant to the conclusion is whether it is true that "all Bs are Zs," which is ignored in the argument.

Note that if the premises were swapped around in either the conclusion or the first co-premise or if the first premise was rewritten to "All Zs can only be Bs" then it would no longer be a fallacy, although it could still be unsound. This also holds for the following two logical fallacies which are similar in nature to the fallacy of the undistributed middle and also non sequiturs.

An example can be given as follows:
  • All men are human.
  • Women are human.
  • Therefore, women are men.

The general argument for Intelligent Design is logically flawed
  • All life is complex
  • A human designer is required for a mechanical watch, which is complex
  • A supernatural designer is required for life

This argument holds no weight. It is a non sequitur fallacy of the undistributed middle.

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