Alternative semantics for unawareness

S. Modica and A. Rustichini (1994, Theory and Decision 37, 107-124) provide d a logic for reasoning about knowledge where agents may be unaware of cert ain propositions. However, their original approach had the unpleasant prope rty, that nontrivial unawareness was incompatible with partitional informat ion structures. More recently, Modica and Rustichini (1999, Games Econ. Beh av. 27, 265-298) have provided an approach that allows for nontrivial unawa reness in partitional information structures. Here it is shown that their a pproach can be viewed as a special case of a general approach to unawarenes s considered by R. Fagin and J. Y. Halpern (1988, Artificial Intelligence 3 4, 39-76). (C) 2001 Academic Press.