Software entities are more complex for their size than perhaps any other human construct, because no two parts are alike (at least above the statement level). If they are, we make the two similar parts into one, a（71）, open or closed. In this respect software systems differ profoundly from computers,buildings, or automobiles, where repeated elements abound. Digital computers are themselves more complex than most things people build; they have very large numbers of states. This makes conceiving, describing, and testing them hard. Software systems have orders of magnitude more （72）than computers do. Likewise, a scaling-up of a software entity is not merely a repetition of the same elements in larger size; it is necessarily an increase in the number of different elements. In most cases, the elements interact with each other in some（73）fashion,and the complexity of the whole increases much more than linearly. The complexity of software is a(an)（74）property, not an accidental one. Hence descriptions of a software entity that abstract away its complexity often abstract away its essence.Mathematics and the physical sciences made great strides for three centuries by constructing simplified models of complex phenomena, deriving properties from the models, and verifying those properties experimentally. This worked because the complexities（75）in the models were not the essential properties of the phenomena. It does not work when the complexities are the essence. Many of the classical problems of developing software products derive from this essential complexity and its nonlinear increases with size. Not only technical problems but management problems as well come from the complexity.
