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Logic and rhetoric
An illicit process (abbreviated IP) and its two sub-forms, the illicit major and the illicit minor, is a logical fallacy in which a flawed conclusion is reached by improper distribution of the terms in the conclusion in the premises.
Distribution is really about how much of the group you're talking about. An undistributed S (subject) means you're talking about some of the group but not all of it. A distributed S means you're talking about the group as a whole. Illicit process is the fallacy that arises from drawing a deductive conclusion about the entire group based on part of the group. This means illicit process is a one-way fallacy. If we have a distributed S in the premise, we can draw an undistributed conclusion because we're drawing a conclusion about part of the group based on all of the group. But we can't draw a conclusion about all based on a few.
An illicit process has two forms. The subject is the major term and the predicate is the minor term. Undistributed will be abbreviated U and distributed will be D but will only be put after the major or minor term to clarify what the fallacy is exactly.
- P1: All M are S (U).
- P2: Some P are not M.
- C1: Some P are not S (D).
- P1: All Middle are Subject (U).
- P2: Some Predicate are not Middle.
- C1: Some Predicate are not Subject (D).
- P1: All M are S.
- P2: All M are P (U).
- C1: All P (D) are S.
- P1: All Middle are Subject.
- P2: All Middle are Predicate (U).
- C1: All Predicate (D) are Subject.
- P1: All dogs are mammals.
- P2: No cats are dogs.
- C1: Therefore, no cats are mammals.
In this argument, the major term is "mammals". This is distributed in the conclusion (the last statement) because we are making a claim about a property of all mammals: that they are not cats. However, it is not distributed in the major premise (the first statement) where we are only talking about a property of some mammals: only some mammals are dogs.
- P1: All cats are felines.
- P2: All cats are mammals.
- C1: All mammals are felines.
The minor term here is mammal, which is not distributed in the minor premise "All cats are mammals," because this premise is only defining a property of possibly some mammals (i.e., that they're cats.) However, in the conclusion "All mammals are felines," mammal is distributed (it is talking about all mammals being felines). It is shown to be false by any mammal that is not a feline; for example, a dog.
In his critique of a creationist article on genetic recombination, Alec MacAndrew identifies several logical flaws including the illicit process of the following form :
- P1: All recombination causes change in DNA sequences.
- P2: All recombination is regulated by cellular mechanisms.
- C1: Therefore, all changes in DNA sequence are regulated by cellular mechanisms.
The flaw is that, while recombination may cause change in DNA sequences, it is not proposed as causing all change in DNA sequences. However, the conclusion is about all DNA sequences. The major premise does not distribute across all DNA sequences while the conclusion does.
An illicit observation is not an error of illicit process. It occurs when two terms that seem to be opposite (but aren't) are used as if they are opposites.
- See the Wikipedia article on Illicit process.
- See the Wikipedia article on Illicit major.
- See the Wikipedia article on Illicit minor.
- Illicit process, illicit major, illicit minor, Fallacy Files
- Syllogistic fallacies, Lander University
- Illicit Observation, Atheism/Agnosticism About.com
- Illicit Minor, Stephen Downes