(Sir Edward) Coke further noted that legal disputes about such matters as inheritance of goods:
are not to be decided by natural reason but by the artificial reason and judgment of law, which law is an art which requires long study and experience, before that a man can attain to the cognisance of it: that the law was the golden metwand and measure to try the causes of the subjects; and which protected his majesty in safety and peace. . . .
In other words, neither a king nor any judge can simply decide idiosyncratically what is ‘fair’. He must instead apply the existing law.
I know that this was written in 1607, but what's the meaning of that? Is it optional? Why or why not?
This answers the question of: How would you determine the purpose of which? It doesn't sound like the modern relative pronoun; "which is an art which" sounds curious. This refers to Why does legal English sometimes repeat the antecedent noun after "which"?.
Source: P13, How the Law Works, Gary Slapper