Definitional fallacies

Cogito ergo sum Logic and rhetoric

Key articles

Logical fallacy Syllogism Argument

General logic

Style over substance Solipsism Exception that proves the rule One-way hash argument Argument from incompatible attributes Appeal to identity

Bad logic

Canard One single proof Appeal to consequences Sunk cost Science was wrong before Wrong direction v - t - e

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[edit]

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

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 that 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.

Ambiguity[edit]

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

External Links[edit]

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