Definitional fallacies

Cogito ergo sum Logic and rhetoric

Key articles

Logical fallacy Syllogism Argument

General logic

Relevance logic Affirmative conclusion from a negative premise Argument from omniscience Formal fallacy Post-designation Hierarchy of disagreement

Bad logic

Friend argument Deepity Appeal to identity Substituting explanation for premise Overprecision Whataboutism v - t - e

Not to be confused with No True Scotsman -- when one redefines a term mid-debate.

Definitional fallacies are logical fallacies that occur when some definition fails to properly explain some term.

Definitional fallacies are fallacies of ambiguity and informal fallacies.

Circularity

See the main article on this topic: Tautology

If a term is defined by itself, the definition carries no new information.

Too broad

The fallacy of "too broad", or "discarded differentia", occurs if when defining a term, instances are included that aren't usually called by that term.

For example:

A fish is an aquatic vertebrate.

This definition would include animals like whales and turtles, which are not fish, so the definition needs to be refined:

A fish is a cold-blooded aquatic vertebrate with two sets of paired fins that is covered with scales.

This definition now excludes those animals that wouldn't be considered fish.

Too Narrow

The fallacy of "too narrow" occurs if when defining a term, instances aren't included that are usually called by that term.

For example:

A rectangle is an object with four perpendicular sides of equal length.

This would exclude all rectangles that aren't square. Instead, rectangle must be defined as:

A rectangle is an object with four sides, each one perpendicular to its adjoining sides.

Ambiguity

See the main article on this topic: Fallacy of ambiguity

A term is ambiguous if it's unclear what it means.

External Links

• Too Broad, Stephen Downes
• Too Narrow, Stephen's Guide