A model means some fundamental elements and some fundamental relations between the elements. The elements could be of any type (physical objects, the representation of any object in any form, including pictures of any type or images of any type or mathematical symbols of any type and so on). In fact, an element could be associated with anything which can be considered as an entity. The elements have some properties, which must be specified somehow. There are a number of relations between the elements, which must also be specified. An image model (or analogic model) contains an unspecified number of elements and an unspecified number of relations between the elements. An image model is just given as it is. It is not possible to specify in explicit and precise ways which are the elements and which are the relations. Examples of image models: maps, models of an object of any type, an assembly of such models including any material elements (water, air and so on), any representation in any form of such elements. A symbolic model uses as elements letters, numbers or words. The relations are of logical or mathematical type. The most important symbolic model is the General Communications Language (GCL). The elements are usually nouns and the relations are usually verbs. Warning: GCL is not really a symbolic model. The GCL just contains all the elements and all the relations. When a symbolic model is made (a sentence, for instance), elements and relations from GCL are used. Thus, because there is no available word, I decided to consider, by extension, the GCL as a symbolic model. In this frame, GCL has to be considered as "symbolic model". Once a model given, it is possible to simulate some situations on it. For simulation, a change must be made to the model. After that, the entire model will be changed because all the elements have some relations between them. Any implicit or explicit information which is generated by simulation by a model, is called "truth". Any truth must be associated with the model, which generated it. This is the definition of the term "truth" in the MDT theory. All the information, which is or could be generated by a model by simulation, is called "reality" associated to that model. This is the definition of the term "reality" in the MDT theory. We also see here that before declaring the reality, one needs to declare the model which generated it. We already used the term "information". This term is a fundamental term. It has no normal definition. MDT accepts the descriptive definition from common life and from science. The same situation is for the term "entity". Warning: in connection with the term "information", something is considered as information after that "something" is processed somehow by a device which takes and processes that "something".