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A Deontic Logic for Two Paradoxes of Deontic Modality (Melissa Fusco, joint work with Arc Kocurek)

The Logic and Metaphysics Workshop will meet on February 10th from 4:15-6:15 in room 7395 of the CUNY Graduate Center for a talk by Melissa Fusco (Columbia).

Title: A Deontic Logic for Two Paradoxes of Deontic Modality

Abstract: In this paper, we take steps towards axiomatizing the two dimensional deontic logic in Fusco (2015), which validates a form of free choice permission (von Wright 1969, Kamp 1973; (1) below) and witnesses the nonentailment known as Ross’s Puzzle (Ross 1941; (2) below).

(1) You may have an apple or a pear ⇒ You may have an apple, and you may have a pear.

(2) You ought to post the letter = ̸⇒ You ought to post the letter or burn it.

Since <>(p or q) = (<>p ∨ <>q) and [ ](p) ⇒ [ ](p ∨ q) are valid in any normal modal logic – including standard deontic logic – the negations of (1)-(2) are entrenched in modal proof systems. To reverse them without explosion will entail excavating the foundations of the propositional tautologies. The resulting system pursues the intuition that classical tautologies involving disjunctions are truths of meaning, rather than propositional necessities. This marks a departure from the commitments the propositional fragment of a modal proof system is standardly taken to embody.

Note: This is joint work with Arc Kocurek (Cornell).

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