Given a model (image or symbolic), it can be used to predict its further evolution. This is achieved by changing/ adding/ removing a parameter/ element/ relationship and following what happens. This process is called simulation on the model. As we know, the results of the simulation on the model are called truths associated to the model. When a model is associated to external reality, by simulating on the model, we can predict the evolution of the external reality. These operations are done either by the human (image and symbolic) or the animal brains (image models only). We need to note here- it is as important, as it looks trivial: We extend to the external reality the structure of symbolic and image models from our brain. This extension is done not only in the domain of science and technology, but also in all domains of life. For each of us, the world itself is given as a sum of all the projections to external reality of all the active models of the brain. This statement is true for animals as well. Example: The laws voted in the parliament are long-range symbolic models; they are an extension of the structure of models from the brains of the authors of the laws. The prediction of the evolution of external reality (see general theory) is the main requirement of design for the human or animal brain. Thus, this requirement is fulfilled by the facility of the brain to build and operate models. ETA 2: Truth, reality, and communication Any result of the simulation on a model is a truth associated to that model. As pointed out in the general theory, a truth is associated by us to a symbolic message (generated by a symbolic model); however in order to keep the terminology simple, in the case of image models, a result obtained by simulation on the model is also called 'truth', in spite of the fact that it is used 'as is', without the necessity to explain it. Example: If an animal builds an image model of the external reality, predicting a dangerous situation, it is possible to find the solution to the problem by simulation on the model. This solution (the truth) might be, e.g. to flee. The truth will activate directly the preexistent action model, which is in this case to flee. We'll refer from now on only to symbolic models. If no model is specified, any truth is nonsense. Example: The truth is ãa car crashed into a wall". This truth might be generated by any of the following models: -accident -test -movie/cartoon -computer game In any of the above models, the specified truth is interpreted differently (it has a different meaning). The theory underlines thus, that the model which generated a truth needs to be specified and accepted before the presentation of the truth. This basic requirement is always met in positive sciences.