joseph halpern ppjmcgncscornelledu consider following line reasoning let p proposition ronald born new york p infer q ronald born united states q infer r possible ronald born new jersey hand p infer possible ronald born new jersey arrived contradiction wrong note answer question familiarity modal logic needed lotfi youre confusing logical consequence consistency true q logical consequence p true r logical consequence q theres logical system im familiar infer r q true r consistent q doesnt mean infer r q dont need modal logic see problem pq infer p p consistent q pq inconsistent q example isomorphic joe wed 06 aug 2008 102902 0400 uai paradox