The concept of natural law is fundamental to political philosophy, ethics, and legal thought. The present article shows that as early as the ancient Greek philosophical tradition, three main ideas of natural law existed, which run in parallel through the philosophical works of many authors in the course of history. The first two approaches are based on the understanding that although equipped with reason, humans are nevertheless still essentially animals subject to biological instincts. The first approach defines natural law as the law of the strongest, which can be observed to hold among all members of the animal kingdom. The second conception presents natural law as the principle of self-preservation, inherent as an instinct in all living...