    # 1999/5/7

    #=========#
    # Context #
    #=========#

    1. Background
    =============
    When I studied the fundamental nature of concept-formation, I focused on
    definitions, and the associated concept-hierarchy.  There are three
    methods of defining a concept, e.g.
        person is either man or woman	# integration
        man is person with sex=male		# differentiation
        Dick is Richard H. McCullough	# identity
    Combining all definitions yields a genus-species concept-hierarchy.
    I call this concept-hierarchy a view.  The phrase
        at view=v
    denotes the concept-hierarchy named v.

    McCarthy's notation
        ist(c,p)
    is written as
        at c  p
    in my KR language.  If the at phrase is omitted, c defaults
    to the current context.  My notation provides a convenient method
    of specifying nested contexts.

    The Situation Semantics notation
        c |= p
    is written as
        at c  p
    in my KR language.

    2. Beliefs and knowledge and context
    ====================================
    A belief is a guess about reality.
    Knowledge is an identification of a fact of reality.
    In other words, knowledge is a sequence of true propositions;
    belief is a sequence of propositions which are either true or false.
    Each person (or agent) has his/her personal beliefs and knowledge.

    For simplicity, I will discuss only my knowledge and my context.
    My context is all of my current knowledge.

    To illustrate what this means, I record my knowledge as
    a sequence of true propositions.
        kn is p0;p1;...pn-1;pn
    If I partition my knowledge between pn-1 and pn,
    the explicit context of pn is
        cx is p0;p1;...;pn-1
    The definition context of pn is
        cd is sequence of definitions of all concepts of kn.
    The implicit context of pn is
        ci is sequence of all propositions logically implied by cx and cd.
    As above, my knowledge and context can be characterized as
    a concept-hierarchy and labeled by a view name.

    3. Truth
    ========
    Propositions can be classified into the following categories.
        axiom            implicit in all facts and knowledge
        percept          internal sense data
        measurement      external data
        definition       based on current knowledge
        conclusion       logically implied by the above propositions
        belief           a guess

    Axiom, percept, measurement are true;
    they are the foundation of all knowledge.
    Definition, conclusion, belief may be true or false.

    4. Actions and events
    =====================
    To integrate actions and events into my KR language, I introduce
    a space-time context.
        at space=s,time=t,view=v  p
    where
        s is space where action/event occurs
        t is time when action/event occurs
        v is the context of knowledge at s,t as defined above

Examples