It may immediately be asked--How is that public opinion to be ascertained? The answer is--By ascertaining the _differences_ in opinion between the present and the past. For this purpose it must be observed, that the views of a past age are easily ascertainable, in matters of law, from theoretical writings, history, and judicial decisions; and these views may be reduced to definition. Modern universal intelligence will either agree or disagree in these views. In the mass of instances it will agree, as progress on such points is at all times slow; and not only will the points of _disagreement_ be few, but they will be salient, striking, and generally of popular notoriety. Present, universal, or international opinion, has therefore two portions. 1. That in which it accords with the views of a past generation, that has become historical. 2. That in which it differs from, or contradicts those views. In the first instance, then, we are to ascertain what _were_ the principles of right and justice, from any materials handed down to us; and if those principles agree with, or support the practical rules recorded by the same, or similar sources of information, such are to be accepted as belonging to the code of the Laws of Nations, as far as those principles are uncontradicted by modern opinion. In the second instance, those differences which may either overrule, add to, or complete the public opinion of a past age, are to be ascertained, (by those in whose hands such decisions rest,) by looking to the _wish_ of nations on these points; and this wish may be exhibited in various ways; either by a universal abandonment of a given law, in its non-execution by any nation whatever, for a length of time; by numerous treaties, to obtain by convention an improvement not yet declared by international tribunals; or by extending to the relations and duties of nations, the improvements in the general principles of right and justice, that are at the time being applied to the concerns of private individuals. The judges of such matters are not to ignore what is going on around them; all necessary knowledge is to be brought into court to discover what is the universal feeling of nations in respect of right and wrong, at the time they decide, and if they see a departure from the past sense of right and wrong, to make the modern, and not the ancient, the fountain of modern law; thence deducing the modern rules.