Keywords: Common Belief, Common
Knowledge, Mutual Belief, Mutual Knowledge, Subjective
Facts, Intersubjective Facts.
Abstract: Though concepts of Common Belief and Common
Knowledge are of utmost importance for all (investigations
of) interpersonal relations, they are nearly nowhere defined
explicitly. Here they are. But, as a first step, only within
the socalled additive approach, i.e., by taking some
concepts of (for sake of simplicity: strong and strictly
lrational) single-person-relative beliefs as their basis.
The resulting concepts of Common Belief and Common Knowledge
are (proved to be) regulated by nearly the same logical
principles as are the individualistc Belief- and
Knowledge-concepts. This makes working with these concepts
quite easy. Finally, the weaker concepts of Mutual Belief
and Knowledge will be defined and compared with Common
Belief and Knowledge.