In common life, the declaration of the model is not always done, and often the model does not even exist in an elaborate and coherent form. Emerging from here a long line of conflicts between individuals, groups or cultural zones, which all have their own reality associated to the same external reality. This can be interpreted as a design deficiency of the brain. Some can compensate this hardware deficiency by software, e.g. the individuals situated on level 3 of consciousness (see the general theory). There is a fairly common situation in external reality when a person states a truth, and then builds the model to support it. This happens usually for persons based on image models only, and when they interact with external reality, they only translate the image truth to a symbolic truth. Such persons are recognisable by their rudimentary logic and their tendency to fragment any discussion to particular sections of the external reality. Such persons can't discuss a single general subject. Exercise: Verify yourself and others on the existence and status of the model, which generated any stated truth. It is known from the general theory that a basic problem in the construction of a model associated to the external reality is that we do not know beforehand the elements of the specific external reality. These elements need to be discovered, and the discovered elements are the only ones we can operate with. Warning: The external reality, as defined in MDT, can't have elements and relationships. The elements and relations appear only in the model associated to the external reality. However, in many statements we will use notions like elements and relationships of the external reality, but these need to be understood as elements and relations of the model associated to the external reality. For the external reality, one can use the term 'entity', which identified by the model will become an element of it. However, we have no word available to associate to external reality in the case of relations. By perfectioning the language, such deficiencies will be solved. A basic requirement for the existence of communication is the existence of a single common model accepted by both parts who want to communicate. Without a common symbolic model, there is no communication, as both sides will have their own list of definition of the terms associated to the words. Usually, communication is done only on symbolic models. However, there are more primitive forms of communication using image models (between people, between humans and animals, between animals). The reality is defined in the general theory as the sum of all truths generated, or possibly generated, by a model. As each person has his/her own collection of models, the reality as understood by each person is different from one person to another.