Translation: Opinion as to law or necessity
"What is the law?" I do not mean anything specific like "what is the law regarding the non-conveyance of the burden of positive covenants?" and I certainly do not mean "where can the law be found?" I do not even really mean "what is law?" All of these questions are reasonably easy to answer (although there is some disagreement around the edges of the last one). No, I mean, if you find yourself in a new situation; one that has never occurred to you before, what is the law then?
Let me give you an example:
Let's say it's four o'clock on a weekday morning. You are woken from a pleasant sleep by a crashing and banging downstairs. Upon investigation you discover a slim man, dressed in black rummaging through your stuff. You are understandably angry and, grabbing a heavy object, sneak up behind him and bash him over the head, knocking him instantly unconscious. Now what do you do? Most people, especially the more materialistic and private among us would feel a compulsion to keep hitting the intruder, but equally, most people, if asked, would acknowledge that such actions are illegal. Even if not, if the person had, for example, merely walked past your house, everyone except possibly the most hardened anarchist would acknowledge that running up to the person and beating them senseless should draw the attention of the law.
It is this appreciation of what the law is, and the conviction that it is binding, that the concept of opinio juris attempts to describe. It is usually seen as one of the two requirements for creating customary law.
Where does it come from?
A very good question.
The conviction that a legal entity is bound to act or refrain from acting by a certain law is usually derived from one of three areas: Common Practice, Extrapolation, or arguably the rather more hazy concept of Natural Law.
State Practice
Opinio juris is most notable in the field of International Law. This is largely because unlike people, states, especially large powerful states, are very difficult to control. It is extremely hard to fine a state that does not want to be fined, unless they acknowledge your jurisdiction to do so, and therefore, when laws are created, they are generally built around what states were doing anyway.
The most oft-used example of state practice creating law is that of the Laws of the Sea. Before there was any advanced international jurisprudence, back when state sovereignty reigned supreme, there was still a general agreement that when at sea, states would behave in a certain way. The most famous example of this is the acknowledgement that all states had the right to try and punish pirates, whether or not the pirates were their own nationals, provided the piracy was committed either in their territorial waters or on the high seas. The reasoning was that the pirates were considered to have the impressive sounding designation of Hostis humani generis or Enemies of all Mankind, and therefore it was in the interests of everyone that they be punished.
The point though, is that because all states did this, there began an impression that it was every state's right to do this, and so, a conviction grew that there was a law allowing them to do so, despite the usual rules regarding territorial and flag jurisdiction.
Extrapolation
The human brain is a very very powerful pattern-spotting computer. It is this ability which allows us to learn language, adopt customs, behave properly, and avoid faux pas. The longer you stay within a system, the more easily you follow their customs. The mark of the truly integrated is how they adapt to a new circumstance; when learning a language, it might be knowing how to correctly spell a word after hearing it for the first time and never seeing it written down; when living in a foreign culture, it might be knowing how to act the first time you greet a lover's parents. The same principle applies to law.
Once you have an idea of how law works, you can guess the sorts of things it will do. This is best illustrated by the concept of a legal loophole. Quite often, the reason loopholes remain undiscovered for so long is because everyone is assuming based on their previous appreciation of how the law works that exploiting the loophole would be illegal. This is opino juris. Interestingly, it can also work in reverse – people will assume because they hear stories of lawyers able to exploit loopholes to enable the law to be gotten around, the law will only ever look precisely at the black-letter. Whether or not this is in fact the case will largely depend on the quality of the attorneys and judges involved, but in theory law follows principles and absurdity should not occur, anyway, that is for another writeup.
Natural Law
The very concept of Natural Law – that good laws are derived from nature – is very controversial; positivist law – laws made by people – is far more popular and widely accepted. According to classical natural lawyers such as Aristotle and Thomas Aquinas, objective moral truths can be reached through pure reason. If this is the case, then it follows that instead of good laws being something that is created by humans, good laws already exist naturally in the universe and are merely discovered. However, if it is the case that all natural law is good then it is manifestly the case that not all law is discovered naturally; there are bad laws. Therefore, a natural lawyer would argue that only some of our current laws are in fact natural. For example, the prohibition on murder exists in one form or another in all existent legal systems, and it is argued that it is derived from the natural actions of a community of people – we are not what Hart called a "suicide cult," we have a natural desire to survive, and as such, we do not usually kill without reason, and from this, we derive laws prohibiting it.
If natural law is indeed what the most important of humanity's laws are derived from, and if opinio juris is a necessary source of law as it is on the international stage, then it would be surprising if the twos' coincidence was unrelated. Those laws that are based on natural law are widely held to be the most important, often attaining the rank of jus cogens- laws that are so important that they can never in any circumstances be denied; the right to self defence for example. This importance is often based on the massively detrimental effect that removing them would have. This latter quality can be appreciated from the moment an opportunity to break the potential law arises, and that quality also naturally creates a perception that legal actors are bound to follow it. This perception is of course opinio juris.
When is it used?
In England and Wales (which I am afraid is my area of expertise, so if you live somewhere else, take this with a pinch of salt) opinio juris does not in fact have any application in domestic law. If you rig your neighbour's house with cameras and microphones because you have a genuine conviction that it is your legal obligation to do so, you might be mentally unsound, but you will not be able to argue that you were acting legally. This is quite similar to the maxim that ignorance of the law is no defence – you cannot drive at eighty miles per hour in a thirty zone just because you happened to be looking the other way every time you passed a roadside speed sign.
However, has already been said, on the International Law stage, opino juris is very important. Article 38.1(b) of the Statute of the International Court of Justice explicitly states that the court may use "international custom, as evidence of a general practice accepted as law." Since according to the judgements in the Lotus case1, "only if such [an act was] based on their [the states] being conscious of a duty to [act] would it be possible to speak of an international custom," opinio juris is accepted as a legitimate source of public international law.
This has proved particularly useful when a new situation requires instantaneous agreement as to what the law is. The classic example here is that of the USSR's launching of Sputnik 1 in 1957. Before that time, no artificial satellite had been put into orbit and as such there was no international law governing whether or not it was legal – in particular whether or not it was legally allowed to pass over other states. Nevertheless, the potential use of orbiting satellites was widely appreciated and even when Sputnik apparently burned up over the United States, there was no criticism of the USSR for acting illegally – all states believing they had a legal duty to allow such an orbit. Thus, it became international customary law that states jurisdiction extends as a maximum, as far as the outer-reaches of the atmosphere.
Conclusions
Obviously this is only a very short introduction to the topic, it is also written by someone who has only just begun studying it. Nevertheless, already, the importance of opinio juris is apparent. Acting because you believe yourself to be legally bound to act in that way is an important feature of the law; personal beliefs, politics and morality do not necessarily feature, though it is arguable that consistency is important. Society works best when people's actions can be broadly predicted, even if their personality is very different or unusual. Of course, a person can be easily threatened and deterred from acting illegally, even if they themselves to not recognise the law, a state can usually only be punished financially (or violently if its small enough), and sometimes not even that is sufficient a deterrent. Therefore, in order for the global community to function, it is even more important then at an international level that states obey rules predictably and consistently. It is, in part, the element of opinio juris that enables this to happen.
References
1PCIK, Series A, No.10 1927, p.18; 4 AD, p153
Sources
Shaw, M N International Law (5th Edition, Cambridge University Press, 2005)
Freeman, M D A Lloyd's Introduction to Jurisprudence (Seventh Edition, Thomson, Sweet & Maxwell 2001)
http://en.wikipedia.org/wiki/Opinio_juris
The lectures of Professor R. Piotrowicz, Professor C Harding, and M. Odello and the Seminars of N. Szablewska