§ 30. The question when retorsion is and when it is not justified is not one of law, and is difficult to answer. The difficulty arises from the fact that retorsion is a means of settling such differences as are created, not by internationally illegal, but by discourteous or unfriendly or unfair and inequitable acts of one State against another, and that naturally the conceptions of discourtesy, unfriendliness, and unfairness cannot be defined very precisely. It depends, therefore, largely upon the circumstances and conditions of the special cases whether a State will or will not consider itself justified in making use of retorsion. In practice States have frequently made use of retorsion in cases of unfair treatment of their citizens abroad through rigorous passport regulations, exclusion of foreigners from certain professions, the levy of exorbitant protectionist or fiscal duties; further, in cases of refusal of the usual mutual judicial assistance, refusal of admittance of foreign ships to harbours, and in similar cases. [Sidenote: Retorsion, how exercised.] § 31. The essence of retorsion consists in retaliation for a noxious act by an act of the same kind. But a State in making use of retorsion is by no means confined to acts of the same kind as those complained of, acts of a similar kind being equally admissible. However, acts of retorsion are confined to acts which are not internationally illegal. And, further, as retorsion is made use of only for the purpose of compelling a State to alter its discourteous, unfriendly, or unfair behaviour, all acts of retorsion ought at once to cease when such State changes its behaviour. [Sidenote: Value of Retorsion.] § 32. The value of retorsion as a means of settling certain international differences consists in its compulsory force, which has great power in regulating the intercourse of States. It is a commonplace of human nature, and by experience constantly confirmed, that evil-doers are checked by retaliation, and that those who are inclined to commit a wrong against others are often prevented by the fear of it. Through the high tide of Chauvinism, Protectionism, and unfriendly feelings against foreign nations, States are often tempted to legislative, administrative, and judicial acts against other States which, although not internationally illegal, nevertheless endanger friendly relations and intercourse within the Family of Nations. The certainty of retaliation is the only force which can make States resist the temptation. III REPRISALS