Simplifications are made, whenever necessary, through assumptions.
A proper balance between the level of model simplification and the representation of reality must be obtained because of the cost-benefit trade-off.
The process of modeling is a combination of art and science.
As a science, there are many standard model classes available, and, with practice, an analyst can determine which one is applicable to a given situation.
As an art, creativity and finesse are required when determining what simplifying assumptions can work, how to combine appropriate features of the model classes, and how to integrate models to obtain valid solutions.