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Definition of Model

At the core of conceptual modeling is the idea of a model. The Wikipedia defines a model as "an abstract (or actual) representation of an object or system from a particular viewpoint".  [ Note: this definition has changed, but I still like the old version ]

There are three parts to the definition above - an object or system, a representation and a viewpoint. It is critical to understand the role of each element of a model.

An Object

An "object or system" is essentially an unknown and perhaps unknowable thing; an object as something in the world to be modeled has potentially an unlimited amount of information associated with it. For instance, a chair as an actual physical object in the world possesses innumerable structural characteristics, such as materials and their strengths, as well as myriad facts about the styles of the chair or its history, for instance who sat upon it ( say, it was your great grandfathers chair and consequently is irreplaceable ). There might be many thousands of possible facts associated with a single chair.

A Representation

Of all the many thousands of characteristics associated with a given object, there may be only a few facts or facets that are of interest to us, a few dozen facts as opposed to many thousands. Whenever we use a chair, our mental representation of the object discards at least 99.999% of all information about chairs and focuses exclusively on the 0.001 % of the characters that effect us, most commonly whether it will hold me up without breaking or tipping over.

A Viewpoint

The representation of an object is further simplified by picking a particular viewpoint according to the need of the moment. For instance, if someone is considering standing on a chair to reach something on a shelf rather than just sitting on it, the fact that the seat is made of woven cane would be the most important aspect of the chair for from that particular viewpoint - it would be more likely to break than a metal chair. Even if it were a fairly substantial chair, the fact that it was your grandfather's chair might make it unusable as a step ladder - far better to make a trip to the basement and get a ladder rather than risk breaking it.

We perform the mental gymnastics of mapping from object to representation to viewpoint effortlessly and naturally. Many people are unaware that they are doing it. However, when one is forced by circumstance ( or inclination ) to try to get a computer to perform commonsense tasks of this sort, it can be a very difficult to accomplish.