Plato, steeped in the Aristotlean worldview of dichotomies and dilemmas, could imagine only two antithetical political systems. He lacked the intellectual underpinnings to conceive of a third possibility – a benign, invariable, and eternal monarchy – a monarchy of laws – a republic.
Although in an earlier dialogue, Plato used the term republic, the word did not mean the same in 380 BC as it means today. The republic Plato envisioned was a monarchy composed of "philosopher kings," who were educated in panacean philosophy to the exclusion of politics and warfare. The modern concept of a republic had to wait for an idea – an idea proposed by Locke in the 1690s and echoed by Rousseau in the 1760s – the idea of the monarchy of the people.
The concept of the monarchy of the people opened the possibility of a new political system – a system in which the people themselves, without top-down coercion, were to determine the structure of their own society. But this new concept demanded a new discipline – personal responsibility.
The basis of a modern republic is self-control – not self control of one's destiny, but self control of one's passions. The control of one's destiny, in fact, results from and is totally dependent for its existence on the control of one's passions. Only by voluntarily adhering to certain rules of conduct can we enjoy the freedom to make our own choices. Like children, we must prove we are responsible before being trusted with responsibility.
A republic rests on the assumption that all its citizens are responsible republicans. You cannot have a free society if everyone is allowed to make up their own rules as they go along. To accomplish that, everyone needs to be clear on what the rules are. So a republic is well served by the codification of an agreed-upon code of conduct by which every republican is to be guided. That codification is called the law.
But, since not all men are responsible republicans, the law itself requires a set of laws to guide and control it. That set of laws is called a Constitution. A Constitution is the charter that defines the duties and the limits of the law. It is the blueprint for the legal operation of the republic. It is the law of laws. While the body of a republic is a self-disciplined people, its soul is its Constitution.
So a republic, in short, is a group of people who have agreed to exercise self-discipline in adhering to certain rules of conduct, or laws, as defined within the framework of a Constitution.
Although republics may incorporate democratic institutions, such as the plebiscite, a republic is not a democracy. Democracies, as Plato pointed out, are dangerous because democrats don't respect the law. Democrats are lazy republicans. They'd rather let someone else tell them what to do and what to think than exercise the discipline and responsibility required to govern themselves.