# Re: Modal Logic

```Hi Eric,
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```
Le 11-août-05, à 01:34, Eric Cavalcanti a écrit :

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```Hi Bruno,

On 8/11/05, Bruno Marchal <[EMAIL PROTECTED]> wrote:

I am having a problem understanding this axiom:

```
```(...) Lob formula (B(Bp->p)->Bp), the main axiom of the modal logic
of self-reference (G)
can be interpreted as showing that some form of honest placebo effect
```
works! But this is something I am still taking with some grain of salt. See the book "Forever Undecided" to see Smullyan exploiting the working
```of some self-fulfilling beliefs.
```
```
Suppose p = "it is raining today"

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B(Bp->p) is true because I believe that if I believe it is raining today it IS raining today, since If I believe it is raining today it is because
```I have gone outside and seen that it is raining today, or I believe my
source of information for that matter.

```
But it doesn't follow from that that I do believe that it is raining today.
```It happens by the way that I don't believe it is raining today, because
I can see a beutiful sun outside.

What's wrong?
```
```

Literaly, it means you are less modest than a Lobian machine!
```
If B(Bp -> p) was true, it would mean that whatever the poof you have that p is true, then p is true. What about dreaming that you have look through the window and see it rains?
```
```
(Remember B is not the "incorrigible" first person. B is really for a scientific third person sharable justification). Of course it makes things still more unbelievable, given that you will tell me that in case you have a proof, it is even more amazing you can be wrong. But then it is like that by incompleteness.
```
```
Now, another remark is that I don't think the B can be used at all for everyday belief which are much more complex due to the long social interaction between us. The everyday beliefs are probably mixture of the logics I described in comp, and which concerns sound machines. But to get physics, this is enough.
```
Bruno

http://iridia.ulb.ac.be/~marchal/

```