Brandon Hopkins

In deontic logic, there is a relatively active literature on Input/Output (I/O) logics, as originally developed by Makinson [18]. In that literature, there is a prevailing impression that, in general, these logics should be viewed as apart from the possible-worlds paradigm, certain embedding results notwithstanding. The goal of this paper is to offer some first steps toward the argument that I/O logics should not be viewed as apart from the possible-worlds paradigm, but rather as convenient systems of natural deduction for reasoning in the first-degree fragments of conditional logics. We use dyadic neighborhood semantics to show this. It is not the construction of I/O logics with neighborhood semantics that is the challenge, but minding the nuances of Jørgensen’s dilemma, and figuring out exactly where the distinguishing features of I/O logics show up in the possible-worlds picture. We begin from a relatively standard presentation of I/O logics, but with a bit of preemptive analysis, and then show the way into the possible-worlds paradigm. We conclude with some considerations of where this analysis leaves us.