All the above-named categories or conceptions or institutions, together with some few others of minor importance, belong to the science of law in general, because they appear in every fully developed system. When, however, we get more into particulars, it becomes increasingly difficult to lay down general doctrines or suggest general rules applicable to all communities, because details must be settled with reference to the needs and usages of a given community, and that which suits one would hardly suit another. Here therefore the Philosophy or Science of Jurisprudence will bid farewell to the student, handing him over to those who teach the law of England or Scotland or France or Russia, as the case may be, and bidding him remember to apply the general principles he has mastered to the criticism of the details which he will thenceforth be occupied in learning. The principles which constitute the Science or Theory of Law in general can be adequately stated within moderate compass. The subject is not a large one, unless a writer spreads himself out in ethics on the one hand or accumulates historical details on the other. Nor is it in the knowledge to be given that the value of the study will chiefly lie; it is rather in the training to use the right methods in the right way. Before he is plunged into details, the student ought to acquire the habit of looking for principles, of analysing terms, of perceiving that legal doctrines have all had their growth from rude beginnings and will change further. These aptitudes will serve him when he enters the domain of technical law, which is a domain less of Reason than of Authority. And authority, though it may be called the reason of the past, rules not because it is reason but because it has the sanction of a past pronouncement. Arguments founded on the reason of things or on the tendency of historical development will avail nothing in practice against a positive rule, whether contained in a statute or deducible from a decided case. Seldom indeed will a judicious advocate invoke either Reason or History, unless perhaps in arguing before the House of Lords a point whereon little authority exists. But in reasoning from decided cases, and even in interpreting statutes, his mastery of the methods already described will stand him in good stead. Nor is it to be forgotten that the judge and the writer of text-books have, each of them, important functions in guiding the development of the law. When a question is to be dealt with regarding which authority is scanty or the decisions are conflicting, a jurist belonging to either of these classes may apply the philosophic habit of mind formed by his theoretic studies to the task of finding a solution which shall be sound and durable, because conformable to principle, and standing in the true line of historical development.