Leech’s 11 modal meanings: (i) possibility (theoretical, factual), (ii) ability, (iii) permission, (iv) exclamatory wish, (v) obligation/requirement, (vi) rules and regulations, (vii) logical necessity, (viii) prediction/predictability, (ix) willingness (weak volition), (x) intention (intermediate volition), (xi) insistence (strong volition) (Leech 1971/1987: 73-104).
Intuitively this principle just says that the same action cannot beboth obligatory and forbidden. Note that as initially described, theexistence of dilemmas does not conflict with PC. For as described,dilemmas involve a situation in which an agent ought to do A,ought to do B, but cannot do both A and B.But if we add a principle of deontic logic, then we obtain a conflictwith PC:
Modal logic, Medieval philosophy, University of Glasgow
ambiguity vs merger: a distinction made by Coates (1983): (i) is (it could be either epistemic ('I infer that…') or deontic ('it is necessary that..)) and this will be determined by inspecting a larger context; (ii) is a (where the modal interpretation is inevitably indeterminate between epistemic ('I infer that…') and deontic ('the producer has a moral obligation to offer a good product')). Palmer (1990: 21-2) questions this. Other terms used in this semantic area are: .
Intensional and Higher-Order Modal Logic.
That “ought” is the dual of permissibility is really alargely overlooked pervasive mistaken bipartisan presuppositionpartially characterizing twentieth century ethical theory and deontic logic.
G. H. von Wright (ed.), An Essay in Deontic Logic and …
Either deontic necessity represents “ought”, in whichcase, its dual does not represent permissibility (and neither does anyother construction in SDL), or permissibility is represented in SDL,but “ought” is inexpressible in it despite the ubiquitousassumption otherwise.
An Essay in Deontic Logic and the General Theory of Action
If von Wright launched deontic logic as an academic specialization,Chisholm's Paradox was the booster rocket that provided the escapevelocity deontic logic needed from subsumption under normal modallogics, thus solidifying deontic logic's status as a distinctspecialization. It is now virtually universally acknowledged thatChisholm was right: the sort of conditional deontic claim expressed in(3) can't be faithfully represented in SDL, nor more generally by acomposite of some sort of unary deontic operator and a materialconditional. This is one of the few areas where there is nearlyuniversal agreement in deontic logic. Whether or not this is becausesome special primitive dyadic deontic conditional isoperating or because it is just that some non-materialconditional is essential to understanding important deontic reasoningis still a hotly contested open question.
Deontic Logic (Stanford Encyclopedia of Philosophy)
As moral dilemmas are typically described, they involve a singleagent. The agent ought, all things considered, to do A,ought, all things considered, to do B, and she cannot do bothA and B. But we can distinguishmulti-person dilemmas from single agent ones. The two-personcase is representative of multi-person dilemmas. The situation is suchthat one agent, P1, ought to do A, a second agent, P2, oughtto do B, and though each agent can do what he ought to do, itis not possible both for P1 to do A and P2 to doB. (See Marcus 1980, p. 122 and McConnell 1988.)Multi-person dilemmas have been called “interpersonal moralconflicts.” Such conflicts are most theoretically worrisome ifthe same moral system (or theory) generates the conflictingobligations for P1 and P2. A theory that precludes single-agent moraldilemmas remains uniquely action-guiding for each agent. But if thatsame theory does not preclude the possibility of interpersonal moralconflicts, not all agents will be able to succeed in discharging theirobligations, no matter how well-motivated or how hard they try. Forsupporters of moral dilemmas, this distinction is not all thatimportant. They no doubt welcome (theoretically) more types ofdilemmas, since that may make their case more persuasive. But if theyestablish the reality of single-agent dilemmas, in one sense theirwork is done. For opponents of dilemmas, however, the distinction maybe important. This is because at least some opponents believe that theconceptual argument against dilemmas applies principally tosingle-agent cases. It does so because the ought-to-do operator ofdeontic logic and the accompanying principles are properly understoodto apply to entities about which decisions can be made. To be clear,this position does not preclude that collectives (such as businessesor nations) can have obligations. But a necessary condition for thisbeing the case is that there is (or should be) a central deliberativestandpoint from which decisions are made. This condition is notsatisfied when two otherwise unrelated agents happen to haveobligations both of which cannot be discharged. Put simply, while anindividual act involving one agent can be the object of choice, acompound act involving multiple agents is difficult so to conceive.(See Smith 1986 and Thomason 1981.) Erin Taylor has recently arguedthat neither universalizability nor the principle that 'ought' implies'can' ensure that there will be no interpersonal moral conflicts (whatshe calls “irreconcilable differences”; Taylor 2011, pp. 182-186).These conflicts would raise no difficulties if morality requiredtrying rather than acting, but such a view is not plausible (Taylor2011, pp. 186-188). Still, moral theories should minimize cases ofinterpersonal conflict (Taylor 2011, pp. 189-190).To the extent thatthe possibility of interpersonal moral conflicts raises an intramuraldispute among opponents of dilemmas, that dispute concerns how tounderstand the principles of deontic logic and what can reasonably bedemanded of moral theories.