ives 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.

Legal standards of conduct appear first in Roman equity. In certain
cases of transactions or relations involving good faith, the formula
was made to read that the defendant was to be condemned to that which
in good faith he ought to give or do for or render to the plaintiff.
Thus the judge had a margin of discretion to determine what good faith
called for and in Cicero's time the greatest lawyer of the day thought
these _actiones bonae fidei_ required a strong judge because of the
dangerous power which they allowed him. From this procedural device,
Roman lawyers worked out certain standards or measures of conduct,
such as what an upright and diligent head of a family would do, or the
way in which a prudent and diligent husbandman would use his land. In
similar fashion English equity worked out a standard of fair conduct
on the part of a fiduciary. Later the Anglo-American law of torts
worked out, as a mea