As this law of human nature is universally binding wherever men exist, it cannot be set aside by the mere circumstances of time and place, whence it results that there is a law of war as well as a law of peace. As this law applies to the commencement of armed conflicts, war is never to be undertaken except to assert rights, and when undertaken is never to be carried on except within the limits of rights. It is true that in the conflict of arms laws must be silent, but only CIVIL laws, which govern in times of peace. Those laws which are PERPETUAL, which spring from the nature of man as man, and not from his particular civil relations, continue even during strife and constitute the laws of war. To deny these, or to disobey them, implies a repudiation of human nature itself and of the divine authority which has invested it with rights and obligations. To disavow the imperative character of these perpetual laws, is to revert to barbarism. It is necessary, however to distinguish between Natural Law, that principle of justice which springs from man's rational nature, and Conventional Law, which results from his agreements and compacts. Natural Law remains ever the same, but institutions change. While the study of abstract justice, apart from all that has its origin in the will or consent of men, would enable us to create a complete system of jurisprudence, there is another source which must not be neglected, since men have established the sanctity of certain rules of conduct by solemn convention. The Law of Nations does not consist, therefore, of a mere body of deductions derived from general principles of justice, for there is also a body of doctrine based upon CONSENT; and it is this system of voluntarily recognized obligations which distinguishes international jurisprudence from mere ethical speculation or moral theory. There are CUSTOMS of nations as well as a universally accepted law of nature, and it is in this growth of practically recognized rules of procedure that we trace the evolution of law international--_jus inter gentes_--as a body of positive jurisprudence. It is evident that the mind of Grotius is continually struggling to establish a science upon this positive basis, and it is this which gives a distinctive character to his effort. The great writers of all ages are cited with a superfluous lavishness, not so much to support his claims by an aggregation of individual opinions--still less to display his erudition, as his critics have sometimes complained--as to give a historic catholicity to his doctrine by showing that the laws he is endeavoring to formulate have, in fact, been accepted in all times and by all men. For this purpose also, he makes abundant use of the great authorities on Roman Law, whose doctrines and formulas were certain to carry conviction to the minds of those whom he desired to convince.