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Single Idea 20791

[from 'fragments/reports' by Chrysippus, in 4. Formal Logic / B. Propositional Logic PL / 2. Tools of Propositional Logic / e. Axioms of PL ]

Full Idea

Chrysippus has five indemonstrables that do not need demonstration:1) If 1st the 2nd, but 1st, so 2nd; 2) If 1st the 2nd, but not 2nd, so not 1st; 3) Not 1st and 2nd, the 1st, so not 2nd; 4) 1st or 2nd, the 1st, so not 2nd; 5) 1st or 2nd, not 2nd, so 1st.

Gist of Idea

Chrysippus has five obvious 'indemonstrables' of reasoning

Source

report of Chrysippus (fragments/reports [c.240 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 07.80-81

Book Reference

'The Stoics Reader', ed/tr. Inwood,B/Gerson,L.P. [Hackett 2008], p.22


A Reaction

[from his lost text 'Dialectics'; squashed to fit into one quote] 1) is Modus Ponens, 2) is Modus Tollens. 4) and 5) are Disjunctive Syllogisms. 3) seems a bit complex to be an indemonstrable.

Related Ideas

Idea 8078 Modus ponens is one of five inference rules identified by the Stoics [Chrysippus, by Devlin]

Idea 1875 Dogs show reason in decisions made by elimination [Chrysippus, by Sext.Empiricus]