It is in Common Law legal systems at least
Deciding what the common law is involves the idea of "discovering" the common law. In many appellate judgements the judges will say "It would make sense if X was the law for the following reasons... but if Y were the law it would have the following disadvantages... I hold that the law is X..."
So why a law is as it is is all part of the this reasoning.
Why a law is, as it is, has traditionally also been used in Common Law systems to interpret statutes.
William Blackstone in Vol. 1 of Commentaries on the Laws of England (1765) - pages 37 and 38 - says:
The fairest and most rational method to interpret the will of the
legislator, is by exploring his intentions at the time when the law
was made, by signs the most natural and probable. And these signs are
either the words, the context, the subject-matter, the effects and
consequence, or the spirit and reason of the law. Let us take a short
view of them all.
- Words are generally to be understood in their usual and most known signification; not so much regarding the propriety of grammar, as
their general and popular use. Thus the law mentioned by Pufendorf
which forbad a layman to lay hands on a priest, was adjudged to extend
to him, who had hurt a priest with a weapon. Again; terms of art, or
technical terms, must be taken according to the acceptation of the
learned in each art, trade, and science. So in the act of settlement,
where the crown of England is limited “to the princess Sophia, and the
heirs of her body, being protestants,” it becomes necessary to call in
the assistance of lawyers, to ascertain the precise idea of the words
“heirs of her body;” which in a legal sense comprise only certain of
her lineal descendants.
- If words happen to be still dubious, we may establish their meaning from the context; with which it may be of singular use to compare a
word, or a sentence, whenever they are ambiguous, equivocal, or
intricate. Thus the proeme, or preamble, is often called in to help
the construction of an act of parliament. Of the same nature and use
is the comparison of a law with other laws, that are made by the same
legislator, that have some affinity with the subject, or that
expressly relate to the same point. Thus, when the law of England
declares murder to be felony without benefit of clergy, we must resort
to the same law of England to learn what the benefit of clergy is: and
when the common law censures simoniacal contracts, it affords great
light to the subject to consider what the canon law has adjudged to be
simony.
- As to the subject-matter, words are always to be understood as having a regard thereto; for that is always supposed to be in the eye
of the legislator, and all his expressions directed to that end.
Thus, when a law of our Edward III. forbids all ecclesiastical persons
to purchase provisions at Rome, it might seem to prohibit the buying
of grain and other victuals; but when we consider that the statute was
made to repress the usurpations of the papal see, and that the
nominations to benefices by the pope were called provisions, we shall
see that the restraint is intended to be laid upon such provisions
only.
4. As to the effects and consequence, the rule is, that where words bear either none, or a very absurd signification, if literally
understood, we must a little deviate from the received sense of them.
Therefore the Bolognian law, mentioned by Pufendorf, which enacted
“that whoever drew blood in the streets should be punished with the
utmost severity,” was held after a long debate not to extend to the
surgeon, who opened the vein of a person that fell down in the street
with a fit.
5. But, lastly, the most universal and effectual way of discovering the true meaning of a law, when the words are dubious, is by
considering the reason and spirit of it; or the cause which moved the
legislator to enact it. For when this reason ceases, the law itself
ought likewise to cease with it. An instance of this is given in a
case put by Cicero, or whoever was the author of the treatise
inscribed to Herennius. There was a law, that those who in a storm
forsook the ship, should forfeit all property therein; and that the
ship and lading should belong entirely to those who staid in it. In a
dangerous tempest all the mariners forsook the ship, except only one
sick passenger, who by reason of his disease was unable to get out and
escape. By chance the ship came safe to port. The sick man kept
possession, and claimed the benefit of the law. Now here all the
learned agree, that the sick man is not within the reason of the law;
for the reason of making it was, to give encouragement to such as
should venture their lives to save the vessel: but this is a merit,
which he could never pretend to, who neither staid in the ship upon
that account, nor contributed anything to its preservation. From this
method of interpreting laws, by the reason of them, arises what we
call equity; which is thus defined by Grotius “the correction of
that, wherein the law (by reason of its universality) is deficient.”
For since in laws all cases cannot be foreseen or expressed, it is
necessary, that when the general decrees of the law come to be applied
to particular cases, there should be somewhere a power vested of
defining those circumstances, which (had they been foreseen) the
legislator himself would have expressed. And these are the cases
which, according to Grotius, “lex non exacte definit, sed arbitrio
boni viri permittit” [“law does not define exactly, but leaves some
discretion to the wise judge”]. Equity thus depending, essentially,
upon the particular circumstances of each individual case, there can
be no established rules and fixed precepts of equity laid down,
without destroying its very essence, and reducing it to a positive
law. And, on the other hand, the liberty of considering all cases in
an equitable light, must not be indulged too far; lest thereby we
destroy all law, and leave the decision of every question entirely in
the breast of the judge. And law, without equity, though hard and
disagreeable, is much more desirable for the public good, than equity
without law: which would make every judge a legislator, and introduce
most infinite confusion; as there would then be almost as many
different rules of action laid down in our courts, as there are
differences of capacity and sentiment in the human mind.
