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Reply to “Forrester's Paradox”

Published online by Cambridge University Press:  13 April 2010

Richard E. Robinson
Affiliation:
University of British Columbia

Extract

Forrester claims to have shown that a contradiction can be derived from a set of two apparently innocuous moral (or legal) rules together with standard deontic logic (SDL), a principle for adverbial detachment, and a statement of fact. Let l be a system of laws that has the following as immediate consequences:

(1) It is obligatory* that Smith not murder Jones.

(2) It is obligatory; that, if Smith murders Jones, then Smith murders Jones gently.

Type
Interventions
Copyright
Copyright © Canadian Philosophical Association 1986

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References

1 Chellas, Brian, Modal Logic: An Introduction (Cambridge: Cambridge University Press, 1980).CrossRefGoogle Scholar

2 Although Jacquette does not make it explicit that he takes instances of Adv Det to be theorems, to infer line (9), in “Forrester's Paradox”, correctly from lines (6), (7), and (8) of that derivation, he must do so. It seems reasonable to treat instances of Adv Det as theorems.

3 In Chellas' Modal Logic the deontic relation used to define truth for statements of the form oA relative to a possible world is stated differently: “A sentence of the form oA is true at a possible world just in case A is true at each of the world's deontic alternatives. Alternatively, one may picture the set of deontic alternatives to a world functioning collectively as a proposition that represents a standard of obligation: the proposition expressed by oA holds at a world if and only if the proposition expressed by A is entailed by the standard of obligation for the world” (191).

4 It is more correct to say “M” is assigned the value true, so let “‘M’ is true in w” abbreviate “‘M’ is assigned the value true in w”.