Let Bp be the statement "it is believed that p".

Why is ~Bp not equivalent to B~p? in words it amounts of saying that: "it's not believed that p" equivalent to "it's believed that not p".

If we are dealing in a classical two truth valued doxastic logic the above equivalence should be apparent. But you can also say that you don't believe that p and that you don't believe that ~p, in which case in what do you do believe? something all else, q which is not p and not ~p, and if it's intutionistic logic then ~~p isn't equivalent to p, which is quite hard to fathom. (I like this word "fathom", reminds me of phantom of menace...).

You can replace p with anything, God, Jesus, Moses, whatever... :-D

  • 1
    It may easily be that I do not believe P and also that I do not believe ¬P. Even if P is bivalent, I am not required to have a belief about the truth or falsehood of every proposition. I may suspend belief for lack of evidence. In such a case B(P v ¬P) holds, but neither BP nor B¬P.
    – Bumble
    Feb 8 at 14:16
  • @Bumble If you don't believe Pv~P you are in a serious psychological scenario. One can ask which one of them do you believe? I.e do you B(BpvB~p). BTW how is the knowing operator compared to the belief operator? Is there any duality between them like necessary and possible and forall and exists. There's Fitch's Knowability paradox. Are all truth are knowable? I once read that Knowledge is "justified belief", how would you formalize this relation between B and K? I mean I think you need to add J here. But what would be the semantics?! Feb 8 at 15:08
  • Assuming I'm committed to bivalence with respect to the domain of application, I believe P v ¬P, so B(P v ¬P) holds. But I am still able to suspend belief in P, and also in ¬P. I believe one or other is true, but in the absence of evidence, I am not committed to believing either. Knolwedge differs because one can believe things that are false, but knowing P requires that P is true.
    – Bumble
    Feb 8 at 16:41
  • @Bumble in which case, how do we know that we know? Well there seems to be here an endless regression. Feb 8 at 17:00
  • There are lots of analyses of knowledge. Whether KP entails KKP is a controversial principle in epistemic logic.
    – Bumble
    Feb 8 at 17:15

2 Answers 2


Why is ~Bp not equivalent to B~p? in words it amounts of saying that: "it's not believed that p" equivalent to "it's believed that not p".

To believe and not believe are propositional attitudes. Propositional attitudes (a species of proposition) assert claims about the mental states of agents about other propositions. Thus, one can negate both claims about propositional attitudes (perhaps better named attitudinal propositions) and claims about the propositions themselves. An example will clarify.

Speaker 1 I do not believe that grue exists.
Speaker 2 Ah ha, so, you believe grue DOESN'T exist.
Speaker 1 I can understand why you might think that, but I don't know what 'grue' is, have never heard of 'grue', and have no beliefs about 'grue' one way or the other. How am I to believe or disbelieve in something that is meaningless to me?
Speaker 1 Well, 'grue' is a philosopher's color that is green and then at some points becomes blue in order to drive home a philosophical point.
Speaker 2 Well, that's absurd. Now, I certainly don't believe grue exists.

And here then is the difference between the natural language interpretation of ~Bp and B~p. In the first case, the belief doesn't exist (what ontologists call negative existential quantification) and in the second case, the belief does exist, but the belief is of a proposition that contains negative existential quantification. The first statement's assertoric force is directed towards imputed belief, and the latter towards the object of belief. It's not any more complicated than that.

  • My eyes are grue, or are they bleen?! :-) Feb 9 at 16:48
  • @MathematicalPhysicist Sounds an empirical endeavor to administer to your significant other!
    – J D
    Feb 9 at 16:54
  • 1
    significant others... :-D Feb 9 at 17:42

Consider a proposition you've never thought of before, perhaps, "The Yodeling Lords of Darkness reside in the Castle of Pancakes atop Kangaroo Mountain." Granted, you've now heard of this proposition (as of reading this post), but imagine a scenario where you haven't read this post. Now, if you've never entertained such a proposition in your thoughts, then "intuitively," you aren't in a position to believe it. So ~Bp.

However, since you've never entertained this thought, how should you think that B~p? This would be, "The Yodeling Lords of Darkness don't reside in the Castle of Pancakes atop Kangaroo Mountain." But you don't have any opinions about the evil yodelers since you have no thought of them; you perhaps don't even know about the Castle of Pancakes or the Kangaroo Mountain. Now, if there is doxastic closure, you might be thought to implicitly believe all the things entailed by what you explicitly believe, and perhaps there is some obscure hypothetical awareness, within you, of the general possibility of evil yodelers, breakfast-themed fortresses, and mountains named after Australian animals. So perhaps there is some "virtual," subconscious sense in which you have thoughts about these things after all.

But epistemic closure is suspect, and the belief operator is not confined to doxastic logic alone but is of a piece with many a normal epistemic logic. More precisely, the problem of logical omniscience stands atop its own weird mountain in this realm, so to speak, and expresses the problem of doxastic/epistemic closure. So again "intuitively," one need not believe all the formal implications of one's other, noninferential beliefs. And so again, if you never consciously entertain a given proposition, perhaps because said proposition is so random as to never have been filtered out of your inner doxastic aether, it is hard to say that you actively believe or disbelieve in such propositions. But so ~Bp holds even though B~p doesn't, here.


There is a different context in which ~Bp does often go with B~p, however. In terms of linguistic pragmatics rather than logical semantics, if someone says, "I don't believe that [insert some proposition p]," this often carries the pragmatic implication that this person also does actively believe that ~p. For example, if a religious fanatic says, "I don't believe in the Zero-gods," they probably also mean to say, "I believe that religions that worship the Zero-gods are false." So your familiarity with this context might be what is prompting you to question the counterpart moment in doxastic logic. You might be interested in dialogical logic in this connection, then.

  • I read your profile. Quite interesting mambo jambo you got there.... :-) youtube.com/watch?v=E5XDZwIE1WY Feb 9 at 14:50
  • @MathematicalPhysicist I should probably edit my profile. I still believe in the gist of what I say there, but I no longer believe that the powerset question is a single question, but is an indefinite slew of them. The SEP article on philosophy of mathematics says that the definable powerset function has a proven solution quasi-equivalent to CH, but if there are hyperdefinable powerset functions, then these can go to ~CH. I read an essay that seems to give at least an outline of a possible answer to a question of Shelah's about the "meaningfulness" of random ~CH sentences. Feb 9 at 15:52

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