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One of the main principles of the concept-oriented paradigm is that of duality. One of its consequences is that an element is described using two classes: a reference class and an object class. And instead of a single world of objects in OOP, we get a new world consisting of the realm of references and the realm of objects. An object is considered a thing in itself which is not directly accessible. A reference on the other hand is available directly and provides a mechanism for accessing objects. Thus programming and data management is the process of manipulating references which indirectly represent objects. In this case it is important to legalize reference by making them first class citizens. This particularly means that there have to be means for modelling behaviour of objects and references rather than only objects.
In the concept-oriented paradigm this legalization is performed by introducing concepts as a generalization of conventional classes. Concept is a pair consisting of one reference class and one object class. Instances of the reference class exist in the realm of references. They are passed-by-value and do not have their own references. Instances of the object class exist in the realm of objects. They are passed-by-reference using their representatives from the realm of references. It is important that concepts allow us to model objects and references in their inseparable unity.
Having a reference is the primary and necessary thing for anything that wants to exhibit itself as an existing thing. In other words, without a reference it is simply not possible to let others know that this object does really exist. Thus describing an object in OOP via its object class is not enough for this object to be perceptible and hence useful. In OOP this problem is solved by providing a native class of references which is "inherited" by any other object class. In the concept-oriented programming (COP) any object class may declare its own reference class using the new programming construct, called concept. An important thing here is that the two classes are always defined in pairs and hence we can retain their inseparability and mutual dependence. References provide a front end for the objects they represent while objects may possess some behaviour that needs to be associated with the concrete points in space.
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