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Single Idea 13710

[from 'Logic for Philosophy' by Theodore Sider, in 4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / c. System D ]

Full Idea

In D we add to K a new axiom saying that 'what's necessary is possible' (□φ→◊φ), ..and it can then be proved that tautologies are possible and contradictions are not necessary.

Gist of Idea

In D we add that 'what is necessary is possible'; then tautologies are possible, and contradictions not necessary

Source

Theodore Sider (Logic for Philosophy [2010], 6.4.2)

Book Reference

Sider,Theodore: 'Logic for Philosophy' [OUP 2010], p.166

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