This is a metaphorical expression borrowed from mechanics. The rule, in its proper and natural sense, is an instrument by means of which may be drawn from one point to another, the shortest possible line, which is called a straight line.
The rule is a means of comparison in the arts to judge whether the line be straight, as it serves in jurisprudence, to judge whether an action be just or unjust, it is just or right, when it agrees with the rule, which is the law. It is unjust and wrong, when it deviates from it. lt is the same with our will or our intention.
Rules of law are general maxims, formed by the courts, who having observed what is common to many particular cases, announce this conformity by a maxim, which is called a rule; because in doubtful and unforeseen cases, it is a rule for their decision; it embraces particular cases within general principles.