Epistemic modal logic was invented by Finnish philosopher Jaako Hintikka Gentamicin eye drops online without prescription, to represent knowledge and belief (in a book published in 1962), and is now used by computer scientists to model and design systems of autonomous software agents. Gentamicin eye drops no online prescription, It uses modal operators to indicate which propositions are known to which agents.
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I wonder if Rumsfeld worries about those unknown truths that are, in fact, unknowable…
http://plato.stanford.edu/entries/fitch-paradox/#1
The problem with Rumsfeld is that he believes in positive introspection – the rule that Kp implies KKp. He lists known knowns, known unknowns, and unknown unknowns, but not unknown knowns. There are some things that one knows in such a way that one doesn’t even realize that one knows them (automatic presuppositions that are in fact true, for instance).
Also, note that satisfiability for even simple propositional logic is NP-complete. When you add a modality it in general gets harder (PSPACE-complete in the case of S4, it seems), but adding negative introspection gets you back down to NP, so it really is a useful simplifying assumption, even though it’s clearly false. (Of course, if you believe something like what I said above, then even S4 is too strong, but it’s at least a better approximation.)