The aim of International Law is to regulate the behavior between states & international organizations. In a State community maintenance of law and order becomes essential. As a general rule, states are bound to do their utmost to act within the confines of the framework of the rules of International Law. Any state disregarding these general principles of peaceful and cooperative cohabitation between states runs the risk of incurring the disapproval of the fellow states in the community. Such disapproval will hardly ever limit itself to a tag of bad reputation, but could even lead to severe consequences. Professor J.G. Starke defines International Law as “that body of law which is composed for its greater part of the principles and rules of conduct which states feel themselves bound to observe, and therefore, do commonly observe in their relations with each other, …” 
Doctrinal aspects of the Question
Rules of Public International Law do not only include express treaty provisions, but also Customary International Law. Customary Law therefore, constitutes primary source of International Law, and has given origin to many rules that govern relationships between sovereign states. The binding force of International Customary Law rests on the implied consent of states. States express their consent to be bound by a given customary rule through their own actions or practices. The process of custom formation has struggled with the vexing question of how to promote stability and reliance on customary law, while preserving the voluntary support of customary law in the fluid environment of international relations.
Customary law consists of unwritten rules that are taken as having a binding nature on the parties involved. The same principle applies to international law. These laws deemed binding on states, since states consent to abide by these unwritten rules. In fact it is this consent that gives these rules general application. These rules are generally and emerge in case of lacunae in order to regulate as many aspects of international as possible. Whilst express written rules generally only apply to the parties to a treaty, customary law is deemed to have a wider effect as it bind states that unilaterally submit to its regime.
Although, the principle of customary law is difficult to define, yet it is a crucial philosophic basis for the entire edifice of international law. An example of the elusiveness of custom is Pacta Sunt Servanda  , the principle that promises should be kept is the foundation of the law of treaties but there is no fundamental legal source for this concept besides customary law.
Article 38 (1) (b) of the Statute of the International Court of Justice (ICJ)  establishes custom as a source of law for settling disputes. It refers to custom “as evidence of a general practice accepted as law”. It establishes the 2 constitutive elements of a customary rule:
Its acceptance as law (opinion juris).
A general practice, referred to as the objective element, because it is visible.
These two elements have been juridically approved by the International Court of Justice.
In the 1986 Nicaragua Judgment  the court accepted these 2 elements as being fundamental to the formation of a customary rule.
In the Malta vs. Libya  judgment, the court noted that if one had to look for the substance of customary law, then one should primarily look at the actual practice of states and the opinio juris. Custom has to be recognized among states as a genuine practice, which is thus obligatory.
From the formulation of Article 38, one might think that it is actually the general practice accepted as law which provides the evidence for the existence of an international custom. But, since the practice in question is followed by states because it is believed to be already legally obligatory, the source of the obligation must precede the custom to which the practice gives rise.
Thus, the formulation in the Statute serves to emphasize that the substance of this source of international law is to be found in the practice of states. The practice of states in this context embraces not only their external conduct with each other, but is also evidenced by such interval matters as their domestic legislation, judicial decisions, diplomatic dispatches and ministerial statements in Parliament and elsewhere.
It is important to note that an apparent contradiction is inherent in the doctrine of Opinio Juris. This holds that it is a prerequisite for customary law that state officials must be convinced of the binding nature of the law. The paradox has been widely debated by scholars. In any case this apparent paradox has not prevented acceptance of customary law essentially as defined here.
According to many authors, Article 38  of the Statute of the International Court of Justice is the starting point when considering the sources of Public International Law.
One must look to scholars for help in deciphering Article 38’s technical definition of customary law. According to Ian Brownlie  the traditional definition of custom has three elements:
general state practice;
uniform state practice;
such general and uniform state practice as accompanied by a sense of legal obligations, referred to as Opinio Juris  .
Article 24 of the Statute of the International Commission required the Commission “to consider ways and means for making evidence of customary international law more readily available.” Also, the Council of Europe set up a Committee of Experts on the publication of national digests of state practice in the field of public international law.
Evidence of custom or practice is sparse and the value of these sources varies and depends much on circumstances. Publications of these practices are seldom found yet can be traced in diplomatic correspondence, official manuals on legal questions, e.g. manuals on military law, resolutions relating to legal questions in the UN General Assembly. Obviously the most formal source of evidence is treaty law, or even state law.
Example. The Territorial and Contiguous Zone Act set the limit of the territorial waters of Malta at 12 nautical miles. This is evidence of the practice adopted by the Maltese State.
1. Its Acceptance as law (Opinio Juris)
Opinio juris serves to establish the existence of a legal obligation and distinguishes customs from usage. The Statute of the Court refers to a general practice “accepted as law” and “as obligatory”.
Here the emphasis is on the psychological element as a requirement for the formation of customs. The sense of legal obligation, as opposed to motives of courtesy, fairness or morality has to be real so that states in their practices recognize a distinction between obligation (custom) and usage. Usage is thus a general practice which does not reflect a legal obligation.
Even though custom and usage are used synonymously in every day life and language, they have a different meaning for international jurists. They are defined as follows:
Custom – is a clear and continuous habit of doing certain actions which have grown up under the ages of the conviction that these actions are according to international law, obligation or right.
Usage – is a habit of doing certain actions which has grown up without there being the conviction that these actions are, according to international law, obligatory or right. Usage is a term that refers to the period when a practice has not yet crystallized into customary law. It is the twilight zone of custom. Usage is the practice states undertake, but lacks the binding or obligatory nature of opinion juris.
This distinction in international law is not always referred to in this sense. For instance Hall suggests “that custom has hardened into usage”. Yet in our view we would more likely see usage crystallize into custom. However at what stage does usage turn into custom? This question is one of fact, not of theory. Therefore it will be safe to uphold that when and as soon as a line of international conduct, frequently adopted by states, is considered by these states as generally legally obligatory or right, then the rule which may be extracted from such conduct is a rule of customary international law and therefore binding.
How does one distinguish between acts which create a legal obligation and those which do not? Brownlie states that in this regard the ICJ has 2 methods of approach. In many cases the Court is willing to assure existence of an opinio juris based on evidence of general practice, consensus on literature or previous determination of the Court or other international tribunal. Therefore, it settles for opinions of other legal authorities, by which it assures existence of the rule. On the other hand in a (minority of cases) the ICJ has adopted a more rigorous approach. In these cases it called for positive evidence of the rules actually adapted by states. Usage that continues without developing an opinion juris remains a practice with no legal effect. Technically it is referred to as lex lata and lex ferenda. Lex lata are the settled rules of international law. Lex ferenda would be those rules that are in the formation process before crystallizing into law. It could also mean rules as they should be transformed in accordance with policy. The challenge is to determine where does lex ferenda stop and where lex lata commence. However, when usage, which is an international habit graduates into law, then we get the obligation of all states to respect that rule.
Opinio Juris Sive Necessitatis – This psychological element, which has been described by one authority as “the mutual conviction that the recurrence…is the result of a compulsory rule  ” is an essential requirement for the formation of custom.
Recurrence of the usage or practice tends to develop a legitimate expectation that, in similar future situations, the same conduct or the abstention there from will be repeated.
When this expectation evolves further into a general acknowledgement by states that the conduct or the abstention there from is a matter both of right and obligation, the transition from usage to custom may be said to have been completed.
The statute of the ICJ refers to an international custom, as evidence of a general practice accepted as law.
International courts have repeatedly affirmed the critical role played by Opinio Juris in custom. In the 1927 Lotus Case  , the Permanent Court of International Justice considered whether custom had been formed on a question of jurisdiction, after a French ship and a Turkish ship collided, killing eight Turkish nationals. The French vessel sought refuge in Turkey. Turkey began to prosecute the first officer of the French ship, and France disputed Turkey’s jurisdiction to try him. The Court was asked to decide whether Customary International Law prohibited Turkey from trying the French officer. The fact that France had ‘abstained’ from trying him did not indicate custom. The court held that, only if such abstention were based on states’ being conscious of having a duty to abstain, would it be possible to speak of an international custom  .
North Sea Continental Shelf Cases: the Court didn’t presume the existence of opinio juris in the context of the argument that the equidistance basis of delimiting the continental shelf had become customary law at the date of the Geneva Convention, or that the practice of states based on the Convention had produced customary law. It held that since only a few years had passed between the setting up of the Convention (1958) and the case in point (1968), even though the passage of time is not itself a bar to the formulation of a new customary rule, yet an indispensable requirement was lacking – that practice within the time (especially that of those states whose interests are affected) has become both extensive and virtually uniform, and occurred to show the general recognition that the law invokes a legal obligation.
“Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it  .
The need for such a belief, that is, the existence of a subjective element, is implicit in the very notion of the ‘opinio juris sive necessitatis’. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation. The frequency or even habitual character of the acts is not in itself enough” 
In the Nicaragua vs. United States Case (1986), the Court referred to the notion of ‘opinio juris sive necessitatis’ as a subjective element, which it defined as “a belief that this practice is rendered obligatory by the existence of a rule of law requiring it.”
How to establish State Practice
It is commonly held that in the case where the practice under examination consists of declaratory acts, one should first test the states’ position as it declares it to be, and not the intention that the state might not forward to justify such an action.
In the context of the Soviet intervention in Afghanistan, the Soviet Union declared that it acted because it was invited into Afghanistan. Thus we presume that if the Soviet Union is invited to act, its practice is that it will act, even if the intervention consists in a military one. Thus, here we don’t enter into the legality or legitimacy of the matter, but just establish what the practice is by examining the declaration and not deduce the practice from the intention. The opinio juris test should be applied to the practices established from the declaration, that is, to intervene when invited.
Generality of Practice
No particular passage of time is required, even though the passage of time will of course be part of the evidence in the course of the test.
Complete uniformity is not necessary; substantial uniformity is the requirement. The practice need not be identical but at least in substance the practice must be uniform.
Under Maltese legislation, the outer limit of the territorial sea is 12nm from the base lines. If one wished to substantiate whether the Maltese provision established the outer limit of the territorial sea is in conformity with customary international law, then one would have to collect evidence showing the practice. In fact in the collection of this evidence one would identify 10 states that claim an outer limit of 3 miles, 80 states that claim a 12 mile limit and 15 states that claim a limit between 12 and 20 nm, making Malta within the norm.
In our analysis to establish uniformity we would not be able to include the practice of the 3nm claimants. But, what about the practice of the 15 states that do not claim the 12 miles? It is submitted that the practice is relevant for whilst it claims more than 12 miles it is to be assumed that these states would not object to the Maltese 12 nm claim since their own claim is beyond 12 miles. After all we are seeking substantial uniformity.
The leading pronouncement on this issue appears is the Asylum Case.
“The party which relies on custom…must prove that this custom is established in such a manner that it has become binding on the other party…that the rule invoked…is in accordance with a constant and uniform usage practiced by the States in question….”
In this case the rule is that one does not need to be absolute about the general upholding of the norm as it would be enough to find that stability of the rule was upheld for a period of time. This requirement is important in that it enables us to gauge the stability of the proposed practice. A firm stable practice is required, if the unwritten rule is to enjoy security and duration required from rules that are designed to regulate state behavior on a long term basis. Again, when we look for consistency we do not require complete consistency but we require substantial consistency. We expect that the test of consistency demonstrates that the practice which we are recognizing has acquired a legal character emanating from settled practice. This practice must show that it is durable and that states have constantly acted accordingly. This ensures that the unwritten rule has the character of stability and states have the security which enables them to rely on the rule in their relations to other states. The durability of a uniform and consistent practice may be demonstrated if the practice has been undertaken over a considerable period of time. It may be argued that the longer the period, the more pronounced is the stability of the rule for consistency and uniformity over a considerable period of time.
Mandatory – impose a course of action. This nature is determined by the states themselves e.g. shall be beyond 12 mile mark.
Permissive – allows states to act in a variety of ways within set parameters. E.g. shall not extend beyond 12 mile mark. Thus the state is allowed to act in more then one way and still be consistent with the rule.
(c) Widespread Representation
Universality isn’t required, yet there has to be sufficient practice to support the custom. Authors and judges have tried to quantify this number but the problem was which criteria to choose in order determine whether a rule had widespread support. To determine the support of a state one has also to take into account the special practice of states, that is, look at the particular practice and see which states have an interest which is affected by the practice. Setting an a priori limit order to determine if a practice is widespread, is difficult and maybe irrational, as each case requires its own considerations.
The actions of a nation demonstrate Opinio Juris when they follow a legal custom because they believe they are so bound by International Law. International Law scholar Ian Brownlie argues that the International Court of Justice has taken two divergent approaches to Opinio Juris. Under the first approach, called no scrutiny, the court simply assumes Opinio Juris exists if there is uniform state practice. Under the second approach, called strict scrutiny, the court demands positive evidence that Opinio Juris exists.
The importance of Opinio Juris has to be examined not only with regards to case-law, but also with reference to past and current events in which Opinio Juris played an important part.
According to the Opinio Juris requirement, states must act with the belief that the applied practice is undertaken to fulfill an underlying legal obligation, and that the practice is not followed by the state out of convenience or diplomatic courtesy during a certain period of time. This requirement is aimed at insuring that customary law results from a general consensus of states, rather than from an occasional and unqualified convergence of state practice.
One such case is the Legality of the Threat or Use of Nuclear Weapons Advisory Opinion of 8 July 1996. In this advisory opinion it was stated that states which hold the view that the use of nuclear weapons is illegal, have endeavored to demonstrate the existence of a customary rule prohibiting this use. Courts refer to a consistent practice of non-utilization of nuclear weapons by States since 1945 and they would see in that practice the expression of an Opinio Juris on the part of those who possess such weapons.
Nevertheless, the Court did not intend to pronounce itself upon the practice known as the “policy of deterrence”. It notef, that it is a fact that a number of States adhered to the practice during the greater part of the Cold War not to use nuclear weapons and continue to adhere to it. Furthermore, the members of the international community are profoundly divided on the matter of whether non-recourse to nuclear weapons over the past 50 years, constitutes the expression of an Opinio Juris. Under these circumstances the Court does not consider it able to find that there is such an Opinio Juris.
The Court points out that the adoption each year by the General Assembly, by a large majority, requesting the member States to conclude a convention prohibiting the use of nuclear weapons in any circumstance, reveals the desire of a very large section of the international community to take, by a specific and express prohibition of the use of nuclear weapons, a significant step forward along the road to complete nuclear disarmament. The emergence, as lex lata, of a customary rule specifically prohibiting the use of nuclear weapons, as such, is hampered by the continuing tensions between the nascent Opinio Juris on the one hand, and the still strong adherence to the practice of deterrence on the other. Thus, one can see that in this case the accepted definition of Opinio Juris is hindered.
Comments by state officials indicate a certain ‘legal obligation’ to pursue or not such behavior. Customary Law may also be traced in General Assembly resolution or statements (whether unilateral or bilateral). This conduct was observed by the ICJ in the Nicaragua Case of 1986. The resolutions &/or statements that were expressed in the treaty are not to be considered as (reiteration) that result from a treaty, but also as evidence that the state is accepting such norms and therefore proof the importance and validity of ‘ opinio juris’.
Many scholars and jurists are of the opinion that opinio juris should be given priority in custom formation. In fact, these scholars and jurists insist that a rule doesn’t need to be universally followed before it can be anointed as an international customary norm; it just has to be generally, defended as being of a binding nature.
Such an agreement could be used in ‘Anti Personal Landmines’. States should be urged to ( ) the use of landmines but rather they should cease using them (vide Angola case)
Prof. R.R. Baxter  wrote that practice is 2nd to Opinio Juris, “Reliance on a multilateral treaty as evidence of Customary International Law is not conditional on any demonstration that the signatory States have actually observed the norms of the treaty for any length of time. The process of establishing the state of Customary International Law is one of demonstrating what States consider to be the measure of their obligations. The actual conduct of States in their relations with other nations is only a subsidiary means whereby the rules which guide the conduct of States are ascertained.” that Opinio Juris is given priority.
This notion was adopted by the ICJ & ICT of former Yugoslavia in their decisions. Both institutions seem to embrace that custom is the foundation of Opinio Juris.
In the Nicaragua Case of 1986, the court based its judgment on Opino Juris i.e. although the …… intervening states did not follow customary law, their ‘Opinio Juris’ did, so according to the court there was customary law.
The Dusko Tadic Case was decided by the Appeals Chamber where ICTY based its reasoning on Government statements instead of state practice evidence.
In fact Theodor Meron stated that courts applied a number of ways which indicated Opinio Juris e.g.military members.
E.g. HR which unfortunately are frequently breached, are tacitly accepted as custom by international treaty law. With the Lisbon Treaty this HR in the Charter are annexed with the Lisbon.
In the past although HR were not as such part of EU law, were tacitly accepted through the ECJ’s decisions.
In fact Louis Louis Sohn  (a scholar) stated “The Declaration, as an authoritative listing of human rights, has become a basic component of international customary law, binding all states, not only members of the United Nations.”
Some scholar holds that customary law may be altered. This can be eloquently explained by Ted Stein  , “Whereas traditionally, the question of what states ought to do was answered primarily by asking what they have done, for today’s foreign ministry lawyer, the key question is whether to place reliance upon a rule stated in some treaty or resolution. Correspondingly, Opinio Juris is no longer seen as a consciousness that matures slowly over time, but instead as a conviction that instantaneously attaches to a rule believed to be socially necessary or desirable.” 
Thus, Opinio Juris has gained importance, even surpassing practice in significance in the sources of customary international law. With the momentum that Opinio Juris has achieved over the past years one cannot ignore the relevance it achieved and the major role it is playing in the creation of rules of international law.
Courts and scholars sometimes infer it from the existence of a widespread behavioural regularity. But this makes opinio juris redundant with the state practice requirement, which, by assumption, is insufficient by itself to establish customary international law. To avoid this problem, courts and scholars sometimes require independent evidence of opinio juris, such as a statement by a high-level government official, ratification of a treaty that contains a norm similar to the customary international law in question, or an attitude of approval toward a General Assembly resolution. The appropriate conditions for the use of such evidence are still unsettled.
In addition, there is no convincing explanation of the process by which a voluntary behavioural regularity transforms itself into a binding legal obligation. Opinio juris is described as the psychological component of customary international law because it refers to an attitude that states have toward a behavioural regularity. The idea of opinio juris is mysterious because the legal obligation is created by a state’s belief in the existence of the legal obligation. Opinio juris is really a conclusion about a practice’s status as international law; it does not explain how a widespread and uniform practice actually becomes legal binding.