Illicit process

Core claim: A categorical conclusion is derived in which a major or minor term occurs in a distributed position, although it was undistributed in its premise – i.e., the conclusion makes a broader claim about that class than the premise does. This violates a fundamental rule of syllogistic logic.

• The argument is a formal categorical (syllogistic) inference, or is presented as representing such a form.
• A term that appears in the conclusion – the major term (predicate) or the minor term (subject) – occurs in a distributed position in the conclusion but only in an undistributed position in the premise(s) in which it appears.
• The conclusion thereby makes a broader claim about that class than the premises do – e.g., asserting something about all members of a class although the relevant premise referred only to a part of it.

• Distribution by statement type – distributed means the term refers to its entire class:
  • “All S are P”: subject distributed, predicate not
  • “No S is P”: both distributed
  • “Some S are P”: neither distributed
  • “Some S are not P”: subject not, predicate distributed
• Traditionally this is listed as two separate fallacies – illicit major and illicit minor – depending on which term is affected; this site combines them under the general name. When reporting the error, still name the specific term (major or minor) that is overextended.
• This is a formal fallacy: the invalidity follows from the logical form alone, regardless of whether the individual claims happen to be true (the conclusion may coincidentally be correct).

• Every term that is distributed in the conclusion is also distributed in its premise – e.g., “All mammals are animals; All dogs are mammals → ∴ All dogs are animals” is valid: ‘dogs’, the only term distributed in the conclusion, is distributed in its premise as well.
• The inference is statistical, empirical, or inductive rather than a formal categorical deduction (unless it is presented as logically necessary).

• Undistributed middle (MD) – the other distribution rule: the middle term fails to connect because it is undistributed in all its occurrences. Ask: does the error lie with an outer (conclusion) term being overextended (→ illicit process), or with the middle term failing to connect (→ undistributed middle)?
• Fallacies of ambiguity (MD) – a term or statement is ambiguous (e.g., equivocation: the same word means different things in different premises), which can look like an inconsistent scope. Ask: does the term keep one consistent meaning but get a wider scope in the conclusion (→ illicit process), or does its meaning shift, or is the ambiguity elsewhere (→ fallacies of ambiguity)?

Input: “All dogs are mammals; all mammals are animals; therefore, all animals are dogs.”
Output: ‘Animals’ (the minor term) refers to all animals in the conclusion, but only to the mammals that are animals in its premise. The premises merely show that dogs form a subset of animals; they say nothing about whether every animal is a dog.

Input: “All steel is metal; no gold is steel; therefore, no gold is metal.”
Output: ‘Metal’ (the major term) refers to all metals in the conclusion, but only to the steel that is metal in its premise. The premises say nothing about non-steel metals – gold itself being the most obvious counterexample.

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