However, they are a special area with their own laws, which includes both scientifically precise methods and those that have no counterpart in other areas of construction. In all other fields, the engineer deals with the effects of forces on structures made from artificially produced materials such as steel and concrete, or from carefully selected natural materials like wood or stone. Since the properties of these materials can be reliably determined, the problems associated with design processing can almost always be solved through the direct application of theory or the results of model tests. In contrast, any determination and conclusion about the types of soil present in nature involves many uncertainties. In extreme cases, the assumptions underlying a design are questionable.