### EPISTEMIC LOGIC AND INFORMATION UPDATE [chapter]

Alexandru Baltag, Hans P. van Ditmarsch, Lawrence S. Moss
2008 Philosophy of Information
A A GFED @ABC t GFED @ABC u Again, we would supplement the picture with a description of which states have which coin faces showing: H is true at s and u, while T is true at t. Note that we can no longer use the "all-or-nothing" notion of knowledge from the previous section: in the original state s, Amina knows the state cannot be u (since she knows that she doesn't yet know the face of the coin); while in the new state u, Amina knows the state is neither s or t anymore. In other words, Amina
more » ... nnot distinguish between the initial states s and t, but can distinguish between them and the new state u. We illustrate this by using lines to represent the agent's indifference between two (indistinguishable) possibilities: The way to read an assertion like "there is a line between s and t" is as follows: Amina is indifferent in u between the world being as described in s and being described as in t. The