It is important to see how natural law has influenced modern international law as both natural law and international law has its own distinct origins, anarchic in its nature and one being considered immutable while other is reckoned as being modern and fairly susceptible of change. Natural Law also known as (jus naturale) has played a significant role in the entire history of legal science and has occupied a central place in terms of understanding the nature of law and international law in particular. The issues of its essence, origin and scope also the interaction with positive law are essential in determining or discerning whether it has its place in the international legal system and could be the legitimate object of the study of international law. 
The concept of natural law is a mixed bundle of concepts and not a concept of isolation. The concept refers to the general idea of justice. This is why great greek philosopher “plato” student of Socrates start his famous writing “republic” by invoking the question of what is justice in his remarkable masterpiece of work.  Apart from the general idea of justice, natural law may be interpreted as rules and principles deducible from nature or very state of things. Early natural law philosophers such as Socrates thought that this law could be discerned from the human reasoning. He used his famous three why and how strategy to justify his form of theory of natural law. The natural law school of thought believes that people or all human being has an innate idea of what is justice, what is right and wrong and this right and wrong should be the higher form of law is the natural law school of thought.  The concept of natural law also denotes to phenomena that are not expressly denoted as natural law but cannot be explained by reference to a positivist criteria  . This means that everything not laid down by the “statutes” and case laws or “unwritten” could be recognized as natural law, and one may justify the jurisprudence of validity of law, by saying that it was natural law where it is incapable of explanation by reference to the positivist criteria.
In line with these characteristics, the precise definition and parameters of conceiving natural law have being evolving and altering over different periods of history. This has demonstrated the different logical possibilities of viewing natural law that is not least caused by the legal, social, religious or political sentiment at the relevant time. 
In terms of the origin of natural law, natural law is considered to be as a law not laid down by the human authority competent to create the law in a legal system that is the legislature or usually the parliament in the national legal system, and state consent in international law. 
In terms of its essence natural law is regarded as law of natural state or law that is applicable to societies that have not yet established the organized legal community. It may or may not survive after such organized community is established. Another way of perceiving the essence of natural law is the law applicable to nature that is the law regulating the most natural elements of life of human beings as well as other biological creatures. Yet another possibility is to conceive natural law as the law expressing the essence and idea of law, the basic values law is supposed to serve and embody, that is rules expressive of the ideal of justice, or the principles concerning the inherent nature of the relevant legal institutions. Viewed from different angles, natural law may be conceived as paramount and immutable, or as subject to changes whenever the need for this arises in the relevant legal community. It is on occasions conceived either as divine law derived from the will of God or secular law reflective of the nature of law or of legal community 
1.1 The essence of natural law, Importance and introduction to positive law theory.
The essence of natural law demands for an understanding in its interaction with positive law. Based on doctrinal orientation, natural law is perceived as law from which the positive law derives its validity. Or as the law which sets limits to the validity of positive law or a fallback source or a place to return should the positive law has no answer as to how the relevant situation is governed.  Positive law or the laws laid by according to jurisprudence theorists such as Thomas Aquinas in the thirteenth century to Hans Kelsen in the twentieth century has attempted to mean that law enforced by the human courts are positive law. At later stage this developed into definition that positive law is the law laid down by supreme authority of the community. However, almost all the philosophers of positivist school of thought have attempted to differentiate what is law from what law ought to be or what is enforced from the human court system, from what is generally considered fair, just, and reasonable policies which they thought was the fine share of natural law, and natural justice. So they distinctly rejected that the law enforced by the courts as positive law from demands of justice and morality.  Given these logical possibilities natural law is accorded relevance in different historical contexts. In one way or the other relevance of natural law is acknowledged not only by the naturalists but also within those doctrinal trends that do not expressly state their adherence to the natural law doctrine, and even those that on their face are generally opposed to the natural law doctrine. Among the Roman jurists, natural law was viewed as the law derived from the nature of human beings, and as law expressive of the basic ideas of justice. According to C i c e r o, natural law is immutable. In Middle Ages, the divine concept of God-given natural law acquired increasing relevance, especially in the writings of Thomas A q u i n a s who at the same time did not view it as the immutable law. In this period natural law is sometimes made subservient to the reason of State, for instance in terms of the concept of “just war” which, while claiming to restrain States in their recourse to force, effectively leaves them as sole arbiters in determining the justness of war. 
1.2 The influence of natural law in international law as for the theories of eminent jurists and thinkers
The link between the natural law and international law is evident in the writings of Vitoria, where International law is perceived as universal law which restrains the freedom of action of nations in relations with one another. 
For example the European powers are limited in ways they could legally apply to Indian tribes in the Western hemisphere who are protected by natural law. Vitoria’s writings also show that natural law can be manipulated. Vitoria argues in his thesis that natural law not only protects native Indian tribes but also can justify coercing them.  On the other hand Grotius conceives natural law as purely secular law in the context of the international legal system which would be there even if god did not exist. In the classical school of thought of international law, from Grotius onwards natural law is perceived as one of the basic elements and sources of international law. This is because of the Hobessian approach which claims states live in a natural state without any form of government and hence there can be no international law but natural law. Apart from this it could also be due to the well perceived need to elaborate upon some principles of law, justice or equity that should guide states in their relations with each other and above all to locate the growing legally relevant practice the
Of States within that framework of law, justice and equity. On some instances this practice is perceived as merely expressive of the dictates of natural law, and on other instances it is perceived as an element of positive law. Puffendorf observes that absence of the central government over and above States makes positive International law impossible. Consequently, Puffendorf does not accept that there is any law of nations which is not natural law, especially the voluntary law of nations.