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Definitional fallacies

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Part of the series on
Logic and rhetoric
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Key articles
General logic
Bad logic
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.


See the main article on this topic: Tautology

If a term is defined by itself, defining the term as such adds nothing useful.

Too broad[edit]

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 and is covered with scales.

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

Too Narrow[edit]

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.


See the main article on this topic: Fallacy of ambiguity

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

External Links[edit]