Illicit Major

Also Known As: Illicit Process of the Major Term

Formal Fallacy ID: illicit_major

Definition

The illicit major is a formal fallacy in categorical syllogisms where the major term (the predicate of the conclusion) is distributed in the conclusion but not in the major premise. A term is 'distributed' when the premise makes a claim about all members of that category. This violates the rule that a term cannot be distributed in the conclusion if it was not distributed in the premises.

Examples

"All dogs are animals. No cats are dogs. Therefore, no cats are animals." (The major term 'animals' is distributed in the conclusion but not in the major premise, where only some animals -- namely dogs -- are discussed.)

'All vegans avoid animal products. No dedicated carnivores are vegans. Therefore, no dedicated carnivores avoid animal products.' (The major term 'avoid animal products' is distributed universally in the conclusion, but the major premise only says something about vegans avoiding them, not about all who avoid them.)

'All Olympic sprinters are fast runners. No amateur joggers are Olympic sprinters. Therefore, no amateur joggers are fast runners.' (The major term 'fast runners' is distributed in the conclusion but was only partially referenced in the major premise, making the inference invalid.)

Formal Logic Pattern

FOL Pattern

The First-Order Logic formula representing this reasoning pattern's logical structure.

FOL (First-Order Logic) uses quantifiers (∀ = for all, ∃ = there exists), connectives (∧ = and, ∨ = or, ⇒ = implies, ¬ = not), and predicates to capture the essential form of a reasoning pattern. For example, the Ad Hominem fallacy: Person(x) ∧ HasFlaw(x) ⇒ Invalid(Claim(x)). These patterns allow automated verification of logical validity.

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All M are P; Some S are not M; therefore Some S are not P [P undistributed in premise but distributed in conclusion]

Formal Verification: Not formally decidable

Verification Steps

Verification Steps

Binary yes/no questions that an AI must answer to detect a reasoning pattern in a text.

Each of the 452 aspects has verification steps — simple yes/no questions designed to systematically detect whether a pattern appears in a text. For ad hominem: "Does the argument attack a person rather than their claim?" For false dichotomy: "Are only two options presented when more exist?" This ensures consistent, reproducible analysis.

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Binary (yes/no) questions an LLM must answer to identify this aspect:

1

Is the major term distributed in the conclusion?
Type: binary
2

Is the major term also distributed in the major premise?
Type: binary
3

Does the distribution of the major term match between premise and conclusion?
Type: binary

Description

Why It Works

Syllogistic reasoning is difficult to evaluate intuitively, and people tend to judge arguments by whether the conclusion sounds reasonable rather than checking the formal distribution of terms.

How to Counter

Check whether the major term (predicate of the conclusion) makes a claim about all its members that was not established in the premises. Draw a Venn diagram to visualize the relationships.

Also Known As

Illicit Process of the Major Term

Real-World Context

Appears in everyday categorical reasoning, classification debates in science, and legal arguments where categories are misapplied through faulty syllogistic structure.

Related Aspects

Illicit Minor Fallacy of Four Terms (Quaternio Terminorum) Fallacy of Exclusive Premises Illicit Conversion

Hierarchical Context

→ is a Formal Fallacies

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