Doxastic logic

doxastic logic is a modal logic that is concerned with reasoning about beliefs. The term doxastic is derived from the ancient Greek doxa which means 'belief.' Typically, a doxastic logic uses Bx to mean "It is believed that x is the case" and the set $$\mathbb{B}$$ denotes a set of beliefs. In doxastic logic, belief is treated as a modal operator.


 * $$\mathbb{B}$$: {$$b_{1},b_{2},...,b_{n}$$}

There is a complete parallelism between logicians who believe propositions and mathematical systems that prove propositions. Using doxastic logic, one can express the epistemic counterpart of Gödel's incompleteness theorem of metamathematics, as well as Löb's theorem, and other mathematical results in terms of belief.

Types of reasoners

 * Accurate reasoner  :  A reasoner is (always) accurate if he or she never believes any false proposition. (modal axiom T)


 * Bpp


 * Inaccurate reasoner   A reasoner is inaccurate if there exists a proposition for which he or she  believes it and it is not true.


 * (p)Bpp


 * Conceited reasoner : A reasoner is conceited, if he or she believes he or she is never inaccurate. A conceited reasoner will necessarily lapse into an inaccuracy.


 * B((Bp🇦🇩p))


 * Consistent reasoner   : A reasoner shall be defined as consistent if he or she never believes any proposition and its negation. (modal axiom D)


 * (Bp🇦🇩Bp)


 * Normal reasoner   :  A reasoner shall be called normal if whenever he or she believes p, he or she also believes that he or she believes p. (modal axiom 4)


 * BpBBp


 * Peculiar reasoner : A reasoner shall be called peculiar if there is some proposition p such that he or she believes p and also believes that he or she doesn't believe p. Although a peculiar reasoner may seem like a strange psychological phenomenon, a peculiar reasoner is necessarily inaccurate but not necessarily inconsistent.


 * Bp🇦🇩BBp


 * Regular reasoner   : A reasoner shall be called a regular reasoner if his or her belief is distributive over logical operations. (modal axiom K)


 * B(pq)(BpBq)


 * Reflexive reasoner : A reasoner shall be called reflexive if for every proposition p there is some q such that the reasoner believes q≡(Bq→p). And so if a reflexive reasoner of type 4 believes Bp→p, he or she will believe p. This is a parallelism of Löb's theorem for reasoners.


 * Unstable reasoner : A reasoner shall be called unstable if there is some proposition p such that he or she believes that he or she believes p, but doesn't really believe p. This is just as strange a psychological phenomenon as peculiarity, however, an unstable reasoner is not necessarily inconsistent.


 * Stable reasoner : A reasoner shall be called stable if he or she is not unstable. That is, for every p, if he or she believes Bp then he or she believes p. Note that stability is the converse of normality. We will say that a reasoner believes that he or she is stable if for every proposition p, he or she believes BBp→Bp (he or she believes: "If I should ever believe that I believe p, then I really will believe p).


 * BBpBp


 * Modest reasoner : A reasoner shall be called modest if for every proposition p he or she believes $$ Bp \to p $$ only if he or she believes p. A modest reasoner never believes Bp→p unless he or she believes p. Any reflexive reasoner of type 4 is modest. (Löb's Theorem)


 * B(Bp→p)→Bp


 * Queer reasoner : A reasoner shall be called a queer reasoner if he or she is of type G and believes that he or she is inconsistent. But he or she is wrong in this belief!


 * Timid reasoner : A timid reasoner is afraid to believe p if he or she believes $$ Bp \to B\bot $$

Increasing levels of rationality

 * Type 1 reasoner   :  A type 1 reasoner has a complete knowledge of propositional logic i.e, he or she sooner or later believes every tautology (any proposition provable by truth tables) (modal axiom N). Also, his or her set of beliefs (past, present and future) is logically closed under modus ponens. If he or she ever believes p and believes p→q (p implies q) then he or she will (sooner or later) believe q (modal axiom K). This is equivalent to modal system K.


 * $$p \models Bp$$
 * (Bp🇦🇩B(pq))Bq


 * Type 1* reasoner   : A type 1* reasoner believes all tautologies, his or her set of beliefs (past, present and future) is logically closed under modus ponens, and for any propositions p and q, if he or she believes p→q, then he or she will believe that if he or she believes p then he or she will believe q. The type 1* reasoner has a shade more self awareness than a type 1 reasoner.


 * B(pq)B(BpBq)


 * Type 2 reasoner   : A reasoner is of type 2 if he or she is of type 1, and for if every p and q he or she (correctly) believes: "If I should ever believe both p and p→q, then I will believe q." Being of type 1, he or she also believes the logically equivalent proposition: B(p→q)→(Bp→Bq). A type 2 reasoner knows that his or her beliefs are closed under modus ponens.


 * B((Bp🇦🇩B(pq))Bq)


 * Type 3 reasoner   : A reasoner is of type 3 if he or she is a normal reasoner of type 2.


 * Type 4 reasoner    : A reasoner is of type 4 if he or she is of type 3 and also believes that he or she is normal.


 * Type G reasoner : A reasoner of type 4 who believes he or she is modest.

Gödel incompleteness and doxastic undecidability
Let us say, an accurate reasoner is faced with the task of assigning a truth value to a statement posed to him or her. There exists a statement which the reasoner must either remain forever undecided or lose his or her accuracy. One solution is the statement:


 * S: "You will never believe this statement."

If the reasoner ever believes the statement S, it becomes falsified by that fact, making S an untrue belief and hence making the reasoner inaccurate in believing S.

Therefore, since the reasoner is accurate, he or she will never believe S. Hence the statement was true, because that is exactly what it claimed. It further follows that the reasoner will never have the false belief that S is true. The reasoner cannot believe either that the statement is true or false without becoming inconsistent (i.e. holding two contradictory beliefs). And so the reasoner must remain forever undecided as to whether the statement S is true or false.

The equivalent theorem is that for any formal system F, there exists a mathematical statement which can be interpreted as "This statement is not provable in formal system F". If the system F is consistent, neither the statement nor its opposite will be provable in it

Inconsistency and peculiarity of conceited reasoners
A reasoner of type 1 is faced with the statement "You will never believe this sentence." The interesting thing now is that if the reasoner believes that he or she is always accurate, then he or she will become inaccurate. Such a reasoner will reason: "If I believe the statement then it will be made false by that fact, which means that I will be inaccurate. This is impossible, since I'm always accurate. Therefore I can't believe the statement, it must be false."

At this point the reasoner believes that the statement is false, which makes the statement true. Thus the reasoner is inaccurate in believing that the statement is false. If the reasoner hadn't assumed his or her own accuracy, he or she would never have lapsed into an inaccuracy.

It can also be shown that a conceited reasoner is peculiar.

Self fulfilling beliefs
For systems, we define reflexivity to mean that for any p (in the language of the system) there is some q such that q≡(Bq→p) is provable in the system. Löb's theorem (in a general form) is that for any reflexive system of type 4, if Bp→p is provable in the system, so is p.

Inconsistency of the belief in one's stability
If a consistent reflexive reasoner of type 4 believes that he or she is stable, then he or she will become unstable. Stated otherwise, if a stable reflexive reasoner of type 4 believes that he or she is stable, then he or she will become inconsistent. Why is this? Suppose that a stable reflexive reasoner of type 4 believes that he or she is stable. We will show that he or she will (sooner or later) believe every proposition p (and hence be inconsistent). Take any proposition p. The reasoner believes BBp→Bp, hence by Löb's theorem he or she will believe Bp (because he or she believes Br→r, where r is the proposition Bp, and so he or she will believe r, which is the proposition Bp). Being stable, he or she will then believe p.