It is usual to describe law as an aggregate of rules. But unless the word rule is used in so wide a sense as to be misleading, such a definition, framed with reference to codes or by jurists whose eyes were fixed upon the law of property, gives an inadequate picture of the manifold components of a modern legal system. Rules, that is, definite, detailed provisions for definite, detailed states of fact, are the main reliance of the beginnings of law. In the maturity of law they are employed chiefly in situations where there is exceptional need of certainty in order to uphold the economic order. With the advent of legal writing and juristic theory in the transition from the strict law to equity and natural law, a second element develops and becomes a controlling factor in the administration of justice. In place of detailed rules precisely determining what shall take place upon a precisely detailed state of facts, reliance is had upon general premises for judicial and juristic reasoning. These legal principles, as we call them, are made use of to supply new rules, to interpret old ones, to meet new situations, to measure the scope and application of rules and standards and to reconcile them when they conflict or overlap. Later, when juristic study seeks to put the materials of the law in order, a third element develops, which may be called legal conceptions. These are more or less exactly defined types, to which we refer cases or by which we classify them, so that when a state of facts is classified we may attribute thereto the legal consequences attaching to the type. All of these admit of mechanical or rigidly logical application. A fourth element, however, which plays a great part in the everyday administration of justice, is of quite another character.