" For an investigator I it makes only sense to consider an entity M as a substitute of an entity O, if I can (1) establish an analogy, similarity, or similar relation between M and O that permits the substitution of O by M with respect to purpose and means, time-horizon etc. , and (2) I can actually investigate M in the required way . That implies firstly that M does not have the properties of O that prevent I from investigating O. Also other properties of O may not be properties of M. In that sense a model is more abstract than its original. Secondly, a model M will in general have properties that O does not have and that actually aid I in investigating M. What the investigator I does in modeling is formulating a problem P in terms of the original O, translate that problem into a problem P' in terms of M, obtain a solution S' of P' and translate S' into a solution candidate S of P in terms of O. Amt Plastic Model If S is actually a solution of P then the modeling process was successful and can be terminated. If, however, that is not the case then the investigator I can start over with a different problem, model, means of investigation, or relation between model and original. "

"g. , controller updates user's shopping cart) A view uses the model to generate an appropriate user interface (e. g. , view produces a screen listing the shopping cart contents). The view gets its own data from the model (or direct from the data access layer - sometimes there is no domain logic to do). The model should have no direct knowledge of the view. However, the observer pattern can be used to provide some indirection between model and view, allowing the model to notify interested parties of a change. Plastic Motorcycle Model A view object can register itself with the model and listen for changes but the model itself remains view-agnostic. The controller does not pass domain objects (the model) to the view although it might issue a command telling the view to update itself. "