In this paper we present a formal common sense theory of
perception and belief.  We begin with a logical analysis of
perception, and present formal semantics for a modal perception
operator. We then  consider the relationship between perception
and belief, and in particular the question of when perception
should lead to belief change. Our theory is intended to apply to
both to high-level, mental, perception in humans and to perception
at many levels in artificial agents, but especially at the level of
the symbolic interface between a vision system and a belief system.