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Single Idea 9405

[catalogued under 4. Formal Logic / A. Syllogistic Logic / 1. Aristotelian Logic]

Full Idea

Square of Opposition: horizontals - 'contraries' can't both be true, and 'subcontraries' can't both be false; verticals - 'subalternatives' have downwards-only implication; diagonals - 'contradictories' have opposite truth values.

Gist of Idea

Square of Opposition: not both true, or not both false; one-way implication; opposite truth-values

Source

Aristotle (On Interpretation [c.330 BCE], Ch.12-13)

Book Reference

Aristotle: 'Categories and De Interpretatione', ed/tr. Ackrill,J.R. [OUP 1963], p.62


A Reaction

This is still used in modern discussion (e.g. by Stalnaker against Kripke), and there is a modal version of it (Fitting and Mendelsohn p.7). Corners read: 'All F are G', 'No F are G', 'Some F are G' and 'Some F are not G'.

Related Idea

Idea 15879 The Square of Opposition has two contradictory pairs, one contrary pair, and one sub-contrary pair [Harré]