is the field of logic
that is concerned with obligation
, and related concepts. Alternatively, a deontic logic is a formal system that attempts to capture the essential logical features of these concepts. Typically, a deontic logic uses OA
to mean it is obligatory that A
, (or it ought to be (the case) that A
), and PA
to mean it is permitted (or permissible) that A
. The term deontic
is derived from the ancient Greek déon
, meaning, roughly, that which is binding or proper
Pre-History of Deontic Logic
Philosophers from the Indian Mimamsa school
to those of Ancient Greece
have remarked on the formal logical relations of deontic concepts and philosophers from the late Middle Ages
compared deontic concepts with alethic ones. In his Elementa juris naturalis
notes the logical relations between the licitum
, and indifferens
are equivalent to those between the possible
, and contingens
Mally's First Deontic Logic and von Wright's First Plausible Deontic Logic
, a pupil of Alexius Meinong
, was the first to propose a formal system of deontic logic in his Grundgesetze des Sollens
and he founded it on the syntax of Whitehead's and Russell's propositional calculus
. Mally's deontic vocabulary consisted of the logical constants U and ∩, unary connective !, and binary connectives f and ∞.
- * Mally read !A as "A ought to be the case".
* He read A f B as "A requires B" .
* He read A ∞ B as "A and B require each other."
* He read U as "the unconditionally obligatory" .
* He read ∩ as "the unconditionally forbidden".
Mally defined f, ∞, and ∩ as follows:
- Def. f. A f B = A → !B
Def. ∞. A ∞ B = (A f B) & (B f A)
Def. ∩. ∩ = ¬U
Mally proposed five informal principles:
- (i) If A requires B and if B then C, then A requires C.
(ii) If A requires B and if A requires C, then A requires B and C.
(iii) A requires B if and only if it is obligatory that if A then B.
(iv) The unconditionally obligatory is obligatory.
(v) The unconditionally obligatory does not require its own negation.
He formalized these principles and took them as his axioms:
- I. ((A f B) & (B → C)) → (A f C)
II. ((A f B) & (A f C)) → (A f (B & C))
III. (A f B) ↔ !(A → B)
IV. ∃U !U
V. ¬(U f ∩)
From these axioms Mally deduced 35 theorems, many of which he rightly considered strange. Karl Menger
showed that !A ↔ A is a theorem and thus that the introduction of the ! sign is irrelevant and that A ought to be the case if A is the case. After Menger, philosophers no longer considered Mally's system viable. Gert Lokhorst
lists Mally's 35 theorems and gives a proof for Menger's theorem at the Stanford Encyclopedia of Philosophy
under Mally's Deontic Logic
The first plausible system of deontic logic was proposed by G. H. von Wright
in his paper Deontic Logic
in the philosophical journal Mind
in 1951. (Von Wright was also the first to use the term "deontic" in English to refer to this kind of logic although Mally published the German paper Deontik
in 1926.) Since the publication of von Wright's seminal paper, many philosophers and computer scientists have investigated and developed systems of deontic logic. Nevertheless, to this day deontic logic remains one of the most controversial and least agreed-upon areas of logic.
G. H. von Wright did not base his 1951 deontic logic on the syntax of the propositional calculus as Mally had done, but was instead influenced by alethic modal logics
, which Mally had not benefited from. In 1964, von Wright published A New System of Deontic Logic
, which was a return to the syntax of the propositional calculus and thus a significantly return to Mally's system. (For more on von Wright's departure from and return to the syntax of the propositional calculus, see Deontic Logic: A Personal View
and A New System of Deontic Logic
, both by Georg Henrik von Wright.) G. H. von Wright's adoption of the modal logic of possibility and necessity for the purposes of normative reasoning was a return to Leibniz.
Standard deontic logic
In von Wright's first system, obligatoriness and permissibility were treated as features of acts. It was found not much later that a deontic logic of propositions could be given a simple and elegant Kripke-style semantics, and von Wright himself joined this movement. The deontic logic so specified came to be known as "standard deontic logic," often referred to as SDL, KD, or simply D. It can be axiomatized by adding the following axioms to a standard axiomatization of classical propositional logic:
In English, these axioms say, respectively:
- If it ought to be that A implies B, then if it ought to be that A, it ought to be that B;
- If it ought to be that A, then it is permissible that (i.e. not obligatory that not) A.
FA, meaning it is forbidden that A, can be defined (equivalently) as or .
There are two main extensions of SDL that are usually considered. The first results by adding an alethic modal operator in order to express the Kantian claim that "ought implies can":
where . It is generally assumed that is at least a KT operator, but most commonly it is taken to be an S5 operator.
The other main extension results by adding a "conditional obligation" operator O(A/B) read "It is obligatory that A given (or conditional on) B". Motivation for a conditional operator is given by considering the following ("Good Samaritan") case. It seems true that the starving and poor ought to be fed. But that the starving and poor are fed implies that there are starving and poor. By basic principles of SDL we can infer that there ought to be starving and poor! The argument is due to the basic K axiom of SDL together with the following principle valid in any normal modal logic:
If we introduce an intensional conditional operator then we can say that the starving ought to fed only on the condition that there are in fact starving: in symbols O(A/B). But then the following argument fails on the usual (e.g. Lewis 73) semantics for conditionals: from O(A/B) and that A implies B, infer OB.
Indeed one might define the unary operator O in terms of the binary conditional one O(A/B) as , where stands for an arbitrary tautology of the underlying logic (which, in the case of SDL, is classical). Similarly Alan R. Anderson (1959) shows how to define O in terms of the alethic operator and a deontic constant (i.e. 0-ary modal operator) s standing for some sanction (i.e. bad thing, prohibition, etc.): . Intuitively, the right side of the biconditional says that A's failing to hold necessarily (or strictly) implies a sanction.
Dyadic deontic logic
An important problem of deontic logic is that of how to properly represent conditional obligations, e.g. If you smoke (s), then you ought to use an ashtray (a). It is not clear that either of the following representations is adequate:
Under the first representation it is vacuously true that if you commit a forbidden act, then you ought to commit any other act, regardless of whether that second act was obligatory, permitted or forbidden (Von Wright 1956, cited in Aqvist 1994). Under the second representation, we are vulnerable to the gentle murder paradox, where the plausible statements if you murder, you ought to murder gently, you do commit murder and to murder gently you must murder imply the less plausible statement: you ought to murder.
Some deontic logicians have responded to this problem by developing dyadic deontic logics, which contain a binary deontic operators:
- means it is obligatory that A, given B
- means it is permissible that A, given B.
(The notation is modeled on that used to represent conditional probability.) Dyadic deontic logic escapes some of the problems of standard (unary) deontic logic, but it is subject to some problems of its own.
Many other varieties of deontic logic have been developed, including non-monotonic deontic logics, paraconsistent deontic logics, and dynamic deontic logics.
Deontic logic faces Jørgensen's Dilemma. Norms cannot be true or false, but truth and truth values seem essential to logic. There are two possible answers:
- Deontic logic handles norm propositions, not norms;
- There might be alternative concepts to truth, e.g. validity or success, as it is defined in speech act theory.
- Lennart Åqvist, 1994, "Deontic Logic" in D. Gabbay and F. Guenthner, ed., Handbook of Philosophical Logic: Volume II Extensions of Classical Logic. Kluwer.
- Hilpinen, Risto, 2001, "Deontic Logic," in Goble, Lou, ed., The Blackwell Guide to Philosophical Logic. Blackwell.
- G. H. von Wright, 1951. "Deontic logic," Mind 60: 1-15.