Bruno Marchal wrote: ...

What could this mean in a real world example?## Advertising

Take W as the set of places in Brussels. Take R to be "accessible bywalking in a finite number of foot steps". Then each places at Brusselsis accessible from itself, giving that you can access it with zerosteps, or two steps (forward, backward, ...).Take W as the set of humans, say that aRb if a can see directly, withoutmirror, the back of b. Then a can access all humans except themselves. Ris said to be irreflexive.Another important "concrete" example, which will help us latter to studythe modal logic of quantum logic. Take the worlds to be the vector of anHilbert Space (or of the simpler 3-dimensional euclidian space). Saythat a is accessible to b, i.e. aRb, if the scalar product of a and bis non null (i.e. a and b are not orthogonal).

`These are good illustrative examples, but how do they apply to worlds that just`

`consist of propositions? What is the relation of accessibility in the p,q,r`

`world(s)? Is it negation?`

Brent Meeker