The basis of the object-oriented approach is as good as possible a representation of something that exists in the real world first in a model and later in an IT system. However, this representation will never completely correspond to reality. Everything in the real world, whether it is a living being, an object, or an idea, is so complex and has so many aspects, that this complexity can never be completely represented:
Figure 4.21 A few aspects of Mr. Smith
To allow representation as a model it is necessary to focus on a few particular aspects and to leave out all others. The essential, meaning the interesting, aspects are emphasized and all other aspects are omitted. It is exactly this that is the art of modeling objects.
In order to model objects successfully we have to know for what purpose they are needed in the IT system. The object "Mr. Smith" will look different in a customer management system than in a medical information system or in a tax...