Triangle of opposition

{{Short description|Concept in Aristotelian logic}}

In the system of Aristotelian logic, the triangle of opposition is a diagram{{which|date=July 2024}} representing the different ways in which each of the three propositions of the system is logically related ('opposed') to each of the others. The system is also useful in the analysis of syllogistic logic, serving to identify the allowed logical conversions from one type to another.

In the 19th and 20th centuries, other triangles were proposed, including Nicolai A. Vasiliev's triangle,{{cite web |url=https://philpapers.org/rec/BAZNSF |title=Non-Classical Stems from Classical: N. A. Vasiliev's Approach to Logic and his Reassessment of the Square of Opposition'' (Book review) |last=Bazhanov |first=Valentin |date=January 31, 2008 |website=Philosophy Documentation Center |publisher= |access-date=2024-03-05}} the Jespersenian Triangle, Ginzberg’s triangle of contraries and Sir William Hamilton’s subcontraries.{{cite web |url=https://ling.yale.edu/sites/default/files/files/horn/Histoire_d%27_O_final_.pdf |title=Histoire d'*O: Lexical Pragmatics and the Geometry of Opposition |last=Horn |first=Laurence |date=January 31, 2010 |website=Yale University |publisher= |access-date=2024-03-05}}