What is the contribution of the Greek philosophers to natural law?

[Jurisprudence] The Greek thinkers developed the idea of ‘natural law’ and laid down its essential features. Heraclitus pointed out that the reason is one of the essentials of being. The instability of political institutions and frequent changes in law and government in small city states of Greece made some jurists to think that law was for the purpose of serving the interests of the strong and was a matter of expediency. Socrates was a great inquirer of truth and moral values who emphasised upon practical morality which he based on scientific outlook, ‘virtue’ Plato expounded his concept of ideal state-the republic. Aristotle expounded the theory of natural law of positive law in clearer term than his predecessors. He said that the purpose of state, community and law is to enable man to realize good life that is living according to virtue. His thesis has inspired great philosophers even in modern times. Philosophers like Kant, Hegel, Kelsen and Stammler owe much to him.