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Polish Journal of Philosophy

Volume 6, Issue 2, Fall 2012

Clayton Peterson, Jean-Pierre Marquis
Pages 53-70
DOI: 10.5840/pjphil20126213

A Note on Forrester’s Paradox

In this paper, we argue that Forrester’s paradox, as he presents it, is not a paradox of standard deontic logic. We show that the paradox fails since it is the result of a misuse of (ROM), a derived rule in the standard systems. Before presenting Forrester’s argument against standard deontic logic, we will briefly expose the principal characteristics of a standard system Δ. The modal system KD will be taken as a representative. We will then make some remarks regarding (ROM), pointing out that its use is restricted to the standard system’s theorems, and cannot be applied to contingent conditionals. Finally, we will show that Forrester’s paradox is not a paradox of standard deontic logic, at least not in the sense he intended it to be. We show that the paradox cannot arise in KD since its semantical model is not rich enough to represent the intuitive (informal) validity of the conditional within Forrester’s paradox. We show that the paradox arises within a system that has a finer semantics.