It is in this sense that natural law is taken to be a synonym of moral law. It is that portion of morality which supplies the more important and universal rules for governance of the outward acts of mankind.
In short, the law of nature is written by the fingers of nature in the hearts of mankind. It consists of the rules which nature, personified as a guiding power, is deemed to have evolved and prescribed. Such rules are the principles of justice in its widest sense.
Greek philosophers thought that the material and moral phenomena of the world could be resolved into some simple and general rules and they called them the laws of nature. Salmond distinguishes between natural and positive justice.
According to him, natural justice is the justice as it is indeed and in truth—in its perfect idea. Positive justice is justice as it is conceived, recognised and expressed, more or less incompletely and inaccurately by the civil or some other form of human and positive law.
Natural law is also known as divine law because it is supposed to be the command of God imposed upon men. It is an unwritten law though adopted universally by almost all the human beings of the world.
Some writers have called it as universal or common law because it is of universal validity and is commonly adopted by the mankind.
Natural law is often said lo be the law of Reason. It is so called because Reason dictates the rules of Natural law. It is also known as eternal law because it is uncreated by human agencies and is immutable.
The famous philosopher Aristotle divided law into universal and special, the former consisting of unwritten rules recognised among all men and the latter being the written enactments by which men are governed.