Aggregation
The whole economic system can be represented by a three-industry model if one allows the names of the outputs to be (1) capital, (2) goods, and (3) services. The problem with this representation is that it would not show the influence, say, the textile industry on the ferrous metal industry. This is because both the textile industry and the ferrous metal industry would be contained within a single classification called the "goods industry" and by this process of combining or aggregating these two industries under one system block they would lose their economic individuality.