Generally, ‘nature’ connotes what comes as an inborn characteristic, while ‘convention’ connotes that which is suggested by custom and practice. The opposition between these two terms was an important feature of ancient Greek political thought. Until challenged by the sophists, political thinkers seem to have thought of moral ideas as being natural in the sense that a morally mature person would come to acquire them. The sophist challenge lay in the idea that perhaps moral ideas were human inventions, which were proposed ultimately because they were convenient. A clear statement of this view is presented by Glaucon and Adeimantus in Book 2 of Plato's Republic. People who have both meted out and received injustice ‘began to set down their own laws and compacts and to name what the law commands lawful and just…. [J]ustice…is a mean between what is best—doing injustice without paying the penalty—and what is worst—suffering justice without being able to avenge oneself’. Plato's Socrates devotes the rest of the Republic to arguments designed to rebut this and to show that ideas of justice are indeed natural.
The Greek word we translate as ‘nature’ is physis, from the verb phyein. This shows one of the classic perils of translation. English ‘nature’ is derived from Latin natus, ‘born’. So if something comes naturally to us, the basic connotation is that it is inborn. But Greek phyein has the additional sense ‘make to grow’. Aristotle agreed with Plato that moral qualities were natural, not conventional. But he expresses himself in a biological rather than a metaphysical way when he states ‘Man is by nature a political animal’. This carries the connotation that man grows to full moral maturity only by being the citizen of a Greek polis (city‐state).
The Greek distinction between the natural and conventional has persisted. For instance, those writers who follow the organic analogy are siding with Plato and Aristotle in thinking of the political institutions or ideas they praise as ‘natural’. Those who deny the organic analogy and regard institutions and ideas as human artefacts side with the sophists.