  This paper is aimed at providing a uniform framework for reasoning about
beliefs of multiple agents and their fusion. In the first part of the paper, we
develop logics for reasoning about cautiously merged beliefs of agents with
different degrees of reliability. The logics are obtained by combining the
multi-agent epistemic logic and multi-sources reasoning systems. Every ordering
for the reliability of the agents is represented by a modal operator, so we can
reason with the merged results under different situations. The fusion is
cautious in the sense that if an agent's belief is in conflict with those of
higher priorities, then his belief is completely discarded from the merged
result. We consider two strategies for the cautious merging of beliefs. In the
first one, if inconsistency occurs at some level, then all beliefs at the lower
levels are discarded simultaneously, so it is called level cutting strategy.
For the second one, only the level at which the inconsistency occurs is
skipped, so it is called level skipping strategy. The formal semantics and
axiomatic systems for these two strategies are presented. In the second part,
we extend the logics both syntactically and semantically to cover some more
sophisticated belief fusion and revision operators. While most existing
approaches treat belief fusion operators as meta-level constructs, these
operators are directly incorporated into our object logic language. Thus it is
possible to reason not only with the merged results but also about the fusion
process in our logics. The relationship of our extended logics with the
conditional logics of belief revision is also discussed.
