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 beings. The third approach, also developed in antiquity, shifts the focus to our rationality and develops the idea of natural law as the law of reason within us. Some Christian thinkers who consider the origin of reason in us to be divine, identify the law of reason inherent in us with God's will. This paper gives a brief exposition of the development of these three concepts of natural law in philosophy, with emphasis on the intertwining of these three concepts, which we, however, understand as primarily and essentially independent. The paper concludes with an overview of twentieth-century authors who exclusively focus on only one of the three concepts. The aim of this article is to argue against these one-sided interpretations and to uphold the independence and distinctness of the three historical conceptions of natural law. (shrink)