These rules are very simple; but, practically, the situation of neutrals is a very delicate one, and gives rise to numerous difficulties, for the solution of which, resort must be had to the special treatises on the law of nations. =120. International treaties: their characters: their forms.=--We have seen that nations have among each other, the same as individuals, obligations and rights which they derive from the natural law. But there are other obligations and other rights which are no longer based on nature, but on _special contracts_ or _usages_. The international law which bears on usages is called _customary right_; that which comes from compacts, is called _conventional right_. The compacts between States are called _treaties_. Treaties are _equal_ or _unequal_, according as they promise equal or unequal things; _personal_ or _real_, according as they relate only to certain persons, and during their lives, or as they are independent of persons and last as long as the State itself; _pure_ and _simple_ or _conditional_; in the first case the stipulations are absolute; in the second they depend on certain conditions. There are different species of treaties according to their different objects: treaties of _alliance_; treaties of _boundaries_; treaties of _cession_; treaties of _navigation_ and _commerce_; treaties of _neutrality_; treaties of _peace_. =121. Essential conditions of public treaties.=--As a principle, the rules which govern international compacts are (with the exception of a few differences) the same as those which govern private compacts. There are three fundamental conditions: 1, the consent; 2, a licit cause; 3, the capacity of the contracting parties. (See above, 92.) The _consent_ should be: 1, declared; 2, free; 3, mutual. The licit causes are those which are physically possible or morally legitimate; the illicit causes are those which are contrary to morality, as, for example, would be the establishment of slavery. The capacity of making a compact belongs to the sovereign of the State alone; but it is necessary that this sovereign be really invested with the power. A sovereign stripped of his sovereignty has no power to make compacts, although he might have all the most legitimate rights; and, on the other hand, a usurping power can legitimately make compacts. The reason of this is, that foreign nations are not capable to decide what with another people constitutes the legitimacy or non-legitimacy of power: there is for them, therefore, only the power _de facto_. Yet this is but the general rule. There may be cases where a foreign government may refuse to recognize a usurper's power.