A model is a simplification of reality used to assist in making a decision or a judgment. Everyone uses models because we do not have real cities or economies or ecologies or airplanes on which to experiment.
Most models are mental models in someone s head.
Mental models develop with experience. Once formed, they are difficult to change. They are not easily transferred to others, nor are they easily inspected or assessed by others. Also, when more than a few variables are involved, mental models are very unreliable for making projections because the number of interactions becomes larger than the mind can handle. A. Computer models all derive ultimately from mental models.