A valid syllogism must involve exactly three distinct terms.

Evidence

Three terms (major, middle and minor) must be involved in every valid syllogism. Any categorical syllogism that contains more than three terms is said to commit the formal Fallacy of Four Terms (quaternio terminorem) (see Chapter 10). — Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide to Clear…, 455

Categorization

• DEFINE: DEFINE
• corroborated: 2 independent passages support this claim — Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide to Clear…, 455
• XREF: connects to: A syllogism is an argument containing premises and a conclusion. — Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide to Clear…, 378; Validity of a syllogism relates exclusively to form rather than content — Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide to Clear…, 443; A categorical syllogism must contain exactly three terms used consistently — Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide to Clear…, 839 (+3 more)