The state is individual writ large.

This Statement is coming from Plato's theory of justice. Plato wanted to show that the principles of justice that applied to an individual level can also work the same at the level of the state. In a sense that the state is nothing but a magnified version of an individual.

Just like the 3 elements are organised in a specific manner in a soul, similarly, the state is organised into 3 classes. The basis of classes is in the spiritual quality of the soul. Three classes are ruling class, soldier class and producing class based on their soul dominated by reason, courage and appetite respectively. 

Man is not self-sufficient hence man form a state. Each class plays a vital role in it without this state cannot be sustained. Each class understand the importance of the other class. The essence of Plato's idea of justice is proper stationing. Justice is non-interference in each other work. The difference between state and individual by Plato is similar to the difference between a small letter and a big letter. In the words of Plato, state does not come from oak or rock, state is inherent in the mind of the people living in the state.

The hidden message in Plato's view is that a state is nothing but an idea. State is just the collection of people organically related to each other. Only if people are just, state can be just. Principles of justice are the same for individuals and state.  By this view of Plato, we consider a co-relation between ethics and politics.