This book is not a detailed treatise on international law, but an attempt to stimulate and assist reflection on its principles. It is primarily intended in part performance of a professor's duty to his university, though not without hope that it may be of use to others as well. International law being the science of what a state and its subjects ought to do or may do with reference to other states and their subjects, everyone should reflect on its principles who, in however limited a sphere of influence, helps to determine the action of his country by swelling the volume of its opinion. Indeed to prepare men for the duties of citizenship is the chief justification for introducing into education a subject which, on account of its inevitable defect in precision, is less suited as a training for the mind than as an exercise for the trained mind. Again, international law is not a highly technical subject, and it would be a mistake to aim at giving it more technicality by the I mode of treating it.