The Republic in which Plato presented concept of the state starts with a query. “What is justice,” sitting in his academy Plato asks students who were all experts in their own respective fields.
As per Plato every thing in the world should be given its appropriate place. Biologically human body can be divided in three distinct and incompatible parts. Wisdom comes from head; stomach is responsible for distribution of calories to the whole body through intakes; hands and feet work for the body and act as guards. Humans die when this appropriation is disturbed. Head cannot act in the place of stomach or hand and vice versa.
Like human body he classifies the state population. The philosophers work as head, businessmen, presents and technicians work as stomach and soldiers and administrators work as arms and legs. If one of them takes the place of another, it will be a gross imbalance. With that the state shall fall sick and ultimately die. If each is at its own place, the state will become ideal.