  We analyze a model of interactive unawareness introduced by Heifetz, Meier
and Schipper (HMS). We consider two axiomatizations for their model, which
capture different notions of validity. These axiomatizations allow us to
compare the HMS approach to both the standard (S5) epistemic logic and two
other approaches to unawareness: that of Fagin and Halpern and that of Modica
and Rustichini. We show that the differences between the HMS approach and the
others are mainly due to the notion of validity used and the fact that the HMS
is based on a 3-valued propositional logic.
