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such as stones and chairs are said to exist; events are said to occur or happen or take place (Hacker
1982a). Second, there are differences in the way objects and events are said to relate to space and time.
Ordinary objects have relatively clear spatial boundaries and unclear temporal boundaries; events have
relatively unclear spatial boundaries and clear temporal boundaries. Objects are invidiously located in
space—they occupy their spatial location; events tolerate co-location (Quinton 1979, Hacker 1982b). Ob-
jects can move; events cannot (Dretske 1967). Objects are continuants—they are in time and they persist
through time by being wholly present at every time at which they exist; events are occurrents—they
take up time and they persist by having different parts (or “stages”) at different times (Mellor 1980).
The last distinction is particularly controversial, as there are philosophers who conceive of objects as
four-dimensional entities that extend across time just as they extend across space. Some such philoso-
phers would in fact draw no metaphysically significant distinction between objects and events (Quine
1960). Rather, they would regard the relevant distinction as one of degree: both objects and events would
be species of the same “material inhabitant of space-time” genus (as opposed to the genus “immaterial
inhabitant”, such as the Equator); but whereas events appear to develop quickly in time, objects are
relatively “firm and internally coherent” (Quine 1970). (http://plato.stanford.edu/entries/events/ accessed
2/10/06)
While we do not deny the existence of such controversy in the philosophy literature, we contend
that choosing to represent events as entities at the conceptual level (Chen 1976) naturally supports the
requirements of causality needed to implement active information systems. The parsimony of such
a representation is even more pronounced when the real-world domain modeled by the information
system is artificial rather than natural. While artificial systems such as business organizations have no
special dispensation from natural laws, they often define (1) what events are important, (2) how such
events are identified and characterized, and (3) the rules (or policies) that govern state transitions when
an event occurs. They are concerned not with tracking the effects of natural causes but with ascribing
characteristics (i.e., attribute values) to the affected objects when specified events occur.
events in natural and Artificial Systems
Events may be conceived at multiple levels of abstraction and granularity. In natural (physical) systems,
the processes of nature can be conceived as a series of instantaneous events resulting in incremental
and perhaps unperceivable changes in the states of concrete objects. More usefully, an event can be
conceived as a perceived or intentional occurrence that causes the states of objects to change in predict-
able ways, i.e., according to rules. The goal of science is to determine such rules for events that occur
in the physical world. Newtonian mechanics, for example, can be viewed as (approximate) rules that
govern the changes in the states of physical objects as forces are exerted upon them. Clearly, these rules
are useful at a particular level of abstraction and granularity. For human purposes, the event of interest
may be the application of a force to an object at a point in time and for a specified duration. The states
of interest may be the position of the object or its velocity at other points in time.
An active information system that models a natural domain must include a representation of the
natural laws that govern it. An air-traffic control system, for example, must represent the natural laws
that enable it to predict the positions of the aircraft it is intended to control. However, the events and
associated laws governing state transitions in organizational and societal systems are often artificial,
designed by organizations and societies for their intended purposes (Brooks 1987). Generally Accepted
Accounting Principles, or GAAP (see http://www.fasab.gov/accepted.html), and Sarbanes Oxley (Green
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